So I was my boyfriend's first, so I'm positive he's completely STD free (same with myself). And we've been having sex for about 9 months, but about 3 months ago he started developing a rash on his lower abdomen after sex. The rash is red with some bumps and he says it's itchy. It disappears rather quickly though. Also, this rash began to form once when I was ehem, performing oral sex.
What could this be? It never occured before when we were having sex. I can't think of any reason why this would start.