I don't know how to describe it. It looks like a little bubble of coagulated blood or something. At least, that's what I would guess it is. I've never had anything like it before. Should I be worried? I'm not sexually promiscuous so I don't know. Anybody have any clues what it might be?
I had one once! Blood blister. It just happens, I don't think there's a specific cause. It'll go away, but really slowly. I popped mine because I got impatient and it hurt SO much more after that. I don't recommend it :P