It sounds like you may have been bitten by some sort of insect, perhaps a spider. These are common especially when we are sleeping, and most of the time we never end up knowing what exactly bit us. In certain cases, the bite itself can make you really sick. For example, the brown recluse spider bite can cause tissue death around the bite. These bites are quite rare and your description isn't classic. A more common problem that occurs after a bite is bacterial infection. The bite itself can introduce bacterial into the skin, and scratching it can make it worse. In these cases, people need a short course of antibiotics that are active against the staph bacteria. For these reasons, I think that you should get this lesion on your stomach looked at right away. You can schedule an appointment with your primary care physician
or if you can't get in right away, go to the emergency room. Either way you can have a doctor
examine the area, determine if it is infected, and prescribe the appropriate antibiotic. I also suggest that you track the lesion's growth. You can do this by drawing a line around the edge of it to see if the redness passes the line, or just take photos every day. Good luck.