probably from just scratching yourself while you're sleeping. in your sleep you probably think you have an itch and try hard to get it. uh.. kinda similar to people who bite their nails in the sleep. yeah, wtf. see a fucking doctor, you psychotic fuck.
I've had the same thing happen to me but it usually shows up on my arms and chest. It could be due to the fact that your a little "thick" around the midsection and need to cut a few lbs(no puddin.. i mean pun intended)