When you nose quits breathing well (primary symptom) and when this comes on suddenly it is most likely either a viral infection or an acute allergy flare up. Here are a few clues that might help you decide, and knowing what you have helps know what to expect and how to treat. 

Viral Infection Clues 

If you have been around young children or people who are sick (have the crud), then a viral infection is most likely. Viral infections can differ from illness to illness, so you can't always assume that you know what it feels like. There are hundreds of different viruses and they can have different presentations. 

Viral infections often start in one place, (one side of nose, throat, chest) and then over the course of one or two days spread to the rest of the upper airway. People often produce large amounts of initially clear nasal mucous. 

When in doubt, it is safest to assume that an acute onset of nasal obstruction is from a viral infection. You should assume that you are contagious and wash your hands frequently to reduce that chance of spreading the virus.

Especially when there is some possibility that it is allergic, I would usually treat it as if it is viral but consider adding antihistamines into the treatment plan.

Acute Allergic and Non-Allergic Rhinitis Clues

If you have had some obvious exposure to allergens, then an acute allergy flare up is possible. An acute case of non-allergic rhinitis is also possible. The symptoms are generally short lived once the exposure is eliminated.
Common non-allergic rhinitis exposures 
Perfumes, solvent fumes, smoke, dust 
Allergic and non-allergic rhinitis often has a history of previous similar experiences that are also short lived. Sneezing, itchy red eyes, lack of sore throat, and lack of cough are clues that such an exposure may be to blame. If you do not get this type of airway obstruction frequently and if there is no obvious exposure to some irritant, then allergic and non-allergic rhinitis is not likely the cause.

Chronic sinusitis
Nasal septal deviations
Sinus cysts or polyps
Concha bullosa

Chronic sinusitis

People often mistake this problem for allergic rhinitis but allergic rhinitis doesn't often include pain. Chronic sinusitis can have many different symptoms, congestion is the most common, but chronic sinus pain may be more specific to sinusitis. Symptoms of chronic sinusitis often flare up and cool down. Antibiotics often help as do steroids (decadron shot, medrol dosepak, or prednisone). People can live with some cases of chronic sinusitis without much interference to their daily life. Flare ups of nasal allergies and viral infections are harder on people with chronic sinusitis. The pain from chronic sinusitis can be over the eyes, between the eyes, over the upper teeth, directed at one or both ears, or in the back of the head. It depends on which sinuses are involved as to where the pain feels like it is.

Nasal septal deviations

The bone and cartilage that separates the left and right side of your nasal passage is called the nasal septum. This structure is almost never perfectly straight. When the nasal septum is excessively crooked, it can have sharp spurs of bone that poke into the side wall of the nasal cavity; this is not a rare problem at all. In other cases, a broad area of deviation can touch the side wall. Often there is no history of injury or fracture to explain the deviation. Nasal septal deviations often cause airway obstruction, sometimes on one side, sometimes on both.

Especially when there is a sharp spur of bone, this can cause pain. the pain is usually one sided, it often feels like it is on the side of the face or ear, and it worsens with any causes of nasal congestion. One somewhat helpful diagnostic test that you can do at home is to spray decongestant nasal spray on the side of the pain. If that pain is significantly reduced shortly after spraying, this provides additional evidence of such a abnormality. There is a specific form of para-nasal pain caused by a sharp bone spur called Sluder's Neuralgia.

This type of sinus pain can come and go in severity. The usual first treatment is with steroid nasal sprays and the occasional use of decongestant sprays and oral decongestants. If such treatment is not adequate, you should see a sinus specialist to help confirm the diagnosis and to recommend treatment. A CT scan, X-Ray, or endoscopic exam may be helpful to finalize the diagnosis. Chronic pain from this type of source often benefits from surgical intervention.

Sinus cysts or nasal polyps

Cysts and small polyps within the sinuses are quite common (the most common being a small cyst in the maxillary sinus). In most cases there are no symptoms or problems caused by these benign structures. Sometimes a cyst or polyp obstructs a sinus opening, causing pain secondary to air pressure not equalizing. Usually the cysts are filled with clear or straw colored fluid. Occasionally cysts may contain infected fluid. When they contain infection, they can cause pain that is localize to the area near the cyst itself.

Only with X-Rays or CT scans can one know about sinus cysts and polyps. Even when one is identified, it can be difficult to know if it is a source of pain, in many cases it is not.

Concha bullosa

This structure is a fairly common normal variant. It is the situation when a normally thin plate of bone, the middle turbinate, develops a bubble or small sinus inside the plate. When this occurs, it causes the middle turbinate to be larger than it is supposed to be. The increase in size permits the middle turbinate to forcefully press on surrounding structures and cause pain. It is sort of like 10 pounds in a 5 pound sack.

Pain from a concha bullosa usually presents between the eyes. The pain can be on one side or both. There is probably a correlation between the size of the concha bullosa and its propensity to cause pain. Many small concha bullosa do not cause any problems.

It takes a CT scan to delineate a concha bullosa and to help give some insight into the likely hood that it is a source of sinus pain. Many smaller concha bullosa are totally without problems. Concha bullosa can be treated with steroid nasal sprays and decongestants, or if the symptoms are difficult to control, a minor surgical procedure.

When no other nasal symptoms are present, it is usually best to pursue other causes first. It is uncommon, but not unheard of, for problems that are truly sinus in origin to cause sinus pain and no other nasal symptoms.

For example, if pain is in the upper teeth, a dentist might be considered first. Pain in the ear can be an ear problem or TMJ syndromes. Pain between the eyes or over the eyes can often be a type of headache, even when it feels like the pain originates in the sinuses. In patients with few other symptoms besides pain, I feel that early CT scanning is helpful, so that other options can be considered if the sinuses don't provide an explanation for the pain.

A quick home test can sometimes help clear things up. If 2 squirts of decongestant spray seem to relieve sinus pain, this would point more towards a sinus origin problem. If the decongestant spray is of no benefit, this does not rule out a primary sinus problem. This is a test, not a treatments. Decongestants sprays can only be used for a few days in a row. 

Primary Headaches

Nasal Septal deviations

TMJ Syndrome

Sinusitis
Primary headaches

For patients who only have sinus pain and no other nasal symptoms, it sometimes turns out that no specific sinus problem can be found to explain the pain. Doctors may have treated you for sinus infections, allergies, and other sinus problems but with little success. At some point a doctor will probably get a CT scan to know about those places that you can't see into on physical exam. In many such patients (those with no other nasal symptoms), a definite answer isn't found.

When this becomes the case, the sinus pain is best treated like a headache. All too often, a specialist will tell such a patient "You don't have sinus headaches".   I personally don't think that that is the right approach. So, does a normal brain MRI mean you don't have headaches? Of course not. The same logic goes towards sinus pain in my opinion. Much of this is semantics, but it is important for the doctor to avoid saying things make the patient feel like their problem (pain) is being dismissed.

The way I put it in these cases is "The CT scan doesn't show any areas of infection, obstruction, or anatomic anomaly. I don't think you will get any benefit from antibiotics or allergy medicines or sinus surgery. Let's try treating this with the medicine used for headaches and see if we can get some relief from the pain.  " At this point I usually try a couple of the migraine medicines and refer the patient to a neurologist for a headache workup. Such patients have invariable had many sinus medicines from their primary care provider.  It also turns out that steroids and decongestants can sometimes help headaches when on clear sinus problem is identified.    

The explanation for how your sinuses can hurt when there is no obvious anatomic or structural explanation is that the nerves that supplies the sinus and nasal lining can become irritated or inflamed, for reasons that are often unclear. Perhaps it is the same process that can make the nerves that innervate the meninges of the brain hurt and cause tension and migraine headaches.

Nasal septal deviations

The bone and cartilage that separates the left and right side of your nasal passage is called the nasal septum. This structure is almost never perfectly straight. When the nasal septum is excessively crooked, it can have sharp spurs of bone that poke into the side wall of the nasal cavity; this is not a rare problem at all. In other cases, a broad area of deviation can touch the side wall. Often there is no history of injury or fracture to explain the deviation. Nasal septal deviations often cause airway obstruction, but not always.

Especially when there is a sharp spur of bone, this can cause pain. the pain is usually one sided, it often feels like it is on the side of the face or ear, and it worsens with any causes of nasal congestion. One somewhat helpful diagnostic test that you can do at home is to spray decongestant nasal spray on the side of the pain. If that pain is significantly reduced shortly after spraying, this provides additional evidence of such a abnormality. There is a specific form of para-nasal pain caused by a sharp bone spur called Sluder's Neuralgia.

This type of sinus pain can come and go in severity. The usual first treatment is with steroid nasal sprays and the occasional use of decongestant sprays and oral decongestants. If such treatment is not adequate, you should see a sinus specialist to help confirm the diagnosis and to recommend treatment. A CT scan, X-Ray, or endoscopic exam may be helpful to finalize the diagnosis. Chronic pain from this type of source often benefits from surgical intervention.

TMJ syndrome (Temporo-mandibular joint dysfunction)

I'm not a big believer in this TMJ syndrome thing. Some doctors believe that most temple region and ear region pain is from TMJ syndrome. Certainly there are patients with this problem, but I don't think it should be the default diagnosis for lateral facial pain of uncertain origin.

Certainly if you have pain directly over the jaw joint, if you are a habitual tooth grinder at night, or if you have had recent significant dental work, then this diagnosis should come to mind. If chewing worsens of elicits the pain or if you have had all other options investigated, then it makes sense to entertain TMJ problems as a source for headaches and sinus type pain.

The pain from TMJ syndrom is usually around the ear, temple, or the angle of the jaw bone. It is best evaluated by a dentist who has special interest in this subject. It is treated with simple measures first, and then with a splint that can be purchased OTC or through your dentist.

Concha bullosa and other anatomic abnormalities

This structure is a fairly common normal variant. It is the situation when a normally thin plate of bone, the middle turbinate, develops a bubble or small sinus inside the plate. When this occurs, it causes the middle turbinate to be larger than it is supposed to be. The increase in size permits the middle turbinate to forcefully press on surrounding structures and cause pain. It is sort of like 10 pounds in a 5 pound sack.

Pain from a concha bullosa usually presents between the eyes. The pain can be on one side or both. There is probably a correlation between the size of the concha bullosa and its propensity to cause pain. Many small concha bullosa do not cause any problems.

There are other similar anatomic abnormalities that can cause sinus pain. In all cases, it seems like they are misshapen in a fashion that would have two two adjacent structures apposing when they would normally have an air gap between them. Club shaped middle turbinate, large agger nasi cells, and large ethmoid bullae are some examples.

It takes a CT scan to delineate a concha bullosa and to help give some insight into the likely hood that it is a source of sinus pain. Many smaller concha bullosa are totally without problems. Concha bullosa can be treated with steroid nasal sprays and decongestants, or if the symptoms are difficult to control, a minor surgical procedure.

When no other nasal symptoms are present, it may be most efficient to pursue other causes first. It is uncommon, but not unheard of, for problems that are truly sinus in origin to cause sinus pain and no other nasal symptoms.

For example, if pain is in the upper teeth, a dentist might be considered first. Pain in the ear can be an ear problem or TMJ syndromes. Pain between the eyes or over the eyes can often be a type of headache, even when it feels like the pain originates in the sinuses. In patients with few other symptoms besides pain, I feel that early CT scanning is helpful, so that other options can be considered if the sinuses don't provide an explanation for the pain.

A quick home test can help clear things up somewhat. If 2 squirts of decongestant spray seem to relieve sinus pain, this would point more towards a sinus origin problem. If the decongestant spray is of no benefit, this does not rule out a primary sinus problem. This is a test, not a treatments. Decongestants sprays can only be used for a few days in a row.

Sinusitis

People often think of sinusitis as a cause of sinus pain, and of course it is one of the more common causes. It is uncommon for sinus pain to be the only symptom from sinusitis, but it is not unheard of. If an isolated ethmoid sinus or frontal sinus or spenoid sinus becomes obstructed or infected, it can cause pain without causing nasal congestion or drainage.

The clues that a blocked or infected sinus is the cause of sinus pain, even when there are no other symptoms are:
One sided pain

Pain that is worse when you lean your head down low

Pain that began after a cold or sinus infection, eventhough other symptoms cleared

Pain that clearly responds to antibiotics

It is usually necessary to obtain a CT scan to clearly diagnose such a problem. Plain X-Rays don't often reveal such problems. If blockage and infection was extensive enought to show up on a plain X-Ray, there would almost always be some congestion, airway obstruction, or drainage.