Iphone 4 keeps randomly vibrating!

The other night when I was sleeping my phone woke me up because it kept vibrating. So when I finally woke up enough to see what was wrong with it, I realized the switch was in between ringer and vibrate. I flipped the switch to vibrate, the mode I usually have the phone in, and went back to sleep. It didn't wake me the rest of the night but since then my phone has been vibrating randomly for no reason and it is driving me insane! Does anyone else have this problem or have any suggestions? I've already tried restoring it and starting from scratch, doing a hard restart, and leaving it turned off for a few hours.