What Causes Redness And Itching Below The Penis Head Post Intercourse?

Hi, two nights ago i had unprotected sex with my girlfriend oral and vaginal 1 week before that day we had unprotected vaginal sex and nothing happened but now after the 2 days i have a red rash on my penis, its on the skin just below the head on the side area its red and its kinda mixed between itch and pain it doesn't hurt when i go to the bathroom or anything just regullary.