It's very possible it's from being overly tired. Also, it may be from a sleep disorder. But, be careful about how you bring this up to people! I found out the hard way. I have narcolepsy and part of it is "waking dreams" and hallucinations that are very real. Not just visual, but audio hallucinations. But when people here hallucination their first inclination is either you're on drugs or schizophrenic. So, if you bring it up to doc, tell him that they happen when you're overly tired and they're concerning you.
Jump on the sleep disorders board and check out some stuff there. I think I posted some of the hallucintations I've had on there under something with narcoleptic in the heading, first couple of pages on the board.
Do you have any other weird symptoms? Post them after reading my posts on sleep disorders because I've laid out just about every weird one I've had. If I get time, I will try to find it and tell you where it is.
You're not going crazy! I think the biggest indicator that you're sane is that you have the presence of mind to wonder if you're nuts! LOL Been there, done that, hated it. Good luck and will pop in, in a bit.
If you don't experience anything bad, you'll never appreciate the good when it comes around!!!