What Causes A Brown Spot At The Tip Of Penis Head Post Intercourse?

I think I have a problem. I just sorta had sex with this really tight girl. She's not a virgin. Also I didn't get finished so I just went upstairs and all of a sudden I see a small brown spot at the tip of my penis! When I touched it a little it sorta hurts like a rug burn. I was trying to push it in though she was tight. Is something wrong