I smell things that aren't there, and other people can't seem to smell them

I'm not sure if this is the right place for this. I smell things that aren't there, and other people can't seem to smell them. I've smelled good things like popcorn and sweet wax, and then I've also smelled bad things. Once I could smell blood on my hands when it was obvious that there was no blood. Is this a mind trick or what?