That happens to me sometimes... my sense of smell overal is quite weak, I would even say, distorted. Sometimes people around me say "argh, what a strong smell here, I can't breathe!" or, while we walk by a blossoming tree, everyone goes "aaah" and I'm just like "what are you talking about?"... sometimes, though, I smell things normally, and sometimes only some part of the smell... like... one time my mother burned pancakes or something, and I swear, it smelled like fresh mint tea. I just skip some part of the smell.
And sometimes I get these smells that are from nowhere, sometimes they are something inbetween smell and taste, sometimes they last only a second, like deja vu, sometimes a few days. I think they have nothing to do with outer world though

, they depend more on my mood or psychological state or something like that I guess...
But I'm not sure about your case, if it's a real smell you are actually feeling, perhaps you better check your walls and under bed, you know...