If you don't have a head trauma then most likely nothing serious and there may be treatments available. You can see another thread where there are about 70 people lost their sense of smell and taste bcuz of head trauma and statistically only 1 in 4 people get their senses back.
I had a head trauma too a year and a half back and for me everything smells what is not supposed to be(parosmia).. It is depressing and tough to cope up everyday
You can consult a Neurologist for peace of mind and don't expect too much from them.
If you live in U.S. and if this is bothering you too much then try this clinic in Washington D.C. I called them to inquire and they really do intense testing, MRIs etc.,
http://www.tasteandsmell.com/
I like to go to them at some point when my other symtoms (dizziness, fatigue etc.,) improves.
good luck.
Forget the fancy name, and don't lie awake thinking you have a brain tumor, for gosh sakes! You may also be experiencing an olfactory seizure; whatshisname's right about seeing a neurologist - if you can find a decent one. If you've had seizures in the past or they are just starting, epileptics are hardly ever burned at the stake anymore. Don't mind me, Sweetie, but I just hate it when stupid doctors toss in 'worst case scenarios' when it could be a very simple non-scary answer. What a jerk!
This is known as phantosmia, it could be the result of let's say head trauma, exposure to toxic chemicals or worst case scenario brain tumor(s).
The other possibility is parosmia, where there is an actual smell but your olfactory system is interpreting it differently ex. a rose smells like mustard. This can be caused by a cold or previous damage to the olfactory system.
Best that you book an appointment with a neurologist either way and get it checked out.
Best Regards