Some migraine sufferers may have olfactory auras involving smelling odors not actually present. This odor can be experienced as a smell 'hallucinations' at onset of migraine or it can be present during the migraine headache.

For how long you are suffering from migraine attacks?
Have you, also, noticed that sometimes you are smelling something other than the scent which is present?

I saw this thread and thought you might take a look at the thread on smoke smell. I've heard of several different types of smells that are phantom smells (phantosmia). I've heard of smoke, menthol, vomit, lilacs, and others ranging from pleasant to not so pleasant. Take a look and see if its something your experiencing. They have a lot more details and possible solutions on the other thread. I was experiencing the persistant smoke smell. It looked like there were several causes ranging from migranes to brain tumors to bacterial causes. I tried the solution for the bacterial and so far so good.