Not Medical Advice:
We found several reasons cited that could be the cause of this condition. The most suspected culprit is sinus infection.
A sinus infection gives you a sense of bad smell, maybe ammoniacal or not.
Asthma and allergies can result in chronic fatigue and muscle weakness and thus is more pronounced with any infection. Allergies can increase your sensitivity to smell.
Anecdotal reports say the constant smell of ammonia in the nose can also be can be associated with chronic kidney failure.
It is also being linked to your body metabolizing protein instead of carbs. An article posted at Runtex.com says that ammonia is actually a by-product of protein metabolism. When you run hard and long and your body lacks adequate carbohydrates, your body burns protein for fuel. When this is burned, the distinctive ammonia smell presents itself afterward.
Another possible reason reported is that you're having olfactory hallucinations (phantosmia), which makes you detect smells that aren't really present in your environment.
According to Mayo Clinic, phantosmia may occur after a head injury or upper respiratory infection. It can also be caused by temporal lobe seizures, inflamed sinuses, brain tumors and Parkinson's disease.
Get facts on causes and symptoms of smell disorder from MedicineNet.com.