It could be a staph infection/boil that burst. My husband occasionally gets them on his inner thighs because of the warmth/sweat/rubbing of his underwear? Where on your stomach is it? I have metal dermatitis and with certain jean "buttons" I get a rash that can ooze (it itches though too and takes a while to develop; I cut out the metal button on my current pair of jeans :-) but see if it is where your underwear elastic rubs or something?

I would go to the doctor and have it checked out. But from what I read, if it's a staph infection it can "spread" and more boils can form, etc.