The bedroom scene...confused?

Maybe half way through the film, Paul and Erik have sex. Paul starts saying "ow, ow, ow" and runs to the bathroom. After this, they change the sheets. Can someone explain exactly what happened here? Did he, uh, mess himself or did he just start bleeding? Or was it that he "climaxed" unexpectedly? I wasn't quite sure what the situation was that caused him to abruptly get up so quickly and rush to the bathroom.