What Are Possible Causes Of Unusual "odors" In Your Head?

What are possible causes of unusual "odors" in your head? In other words, I am congested (allergies and/or mild cold), but the taste in my mouth and smells that I detect are not what I would normally experience from post nasal drip or sinus infection. They are almost chemical-like. We live on a farm and I am exposed to chickens - not sure if that has anything to do with it.... It's just weird and incredibly annoying. Has been going on for a while now, but was exacerbated this week by a bad cold. Suggestions?