Phantosmia is a medical condition where you are smelling odors that are not actually present. This experience can also be called an olfactory hallucination. Some of the smells you may smell are cigarette smoke, rubber, chemicals, or something rotting. Clearly this is not a very pleasant experience at all. The interesting thing about phantosmia is that it could be caused by many different things. Some causes could include nasal polyps, colds, allergies or sinus infections. There are also more serious things that could cause phantosmia such as stroke or a brain tumor.
This is something I have experienced since I was young due to migraines. There are periods of time where I constantly smell cigarette smoke, which honestly makes my life a little miserable. But there are things that help such as washing your nose out with a saline, such as using a neti pot. Or nose spray does seem to help as well when it becomes flared up by my allergies.