The head of my penis hurts when i have sex with my girlfriend (if i'm not using a condom) but it's almost always when she is on top of me.

Pain: Communicate this to your partner so that you can make changes to avoid causing more problems. Sexual acts are not meant to cause pain or damage sensitive parts. Sometimes people forget this, and in the heat of the moment may unknowingly hurt their partner. Consider use of lubricants or changes in position or style of sexual activity to not cause more problems.