What Causes Blood On My Penis Head?

hey the other day while i was in the shower with my girlfriend i looked at my girlfriends belly button and i noticed there was blood around it at the time i thought nothing of it cause she has a belly button ring. later that day i used the bathroom and i noticed there was dry blood on the tip on my penis and there was little drops of blood in my boxers. I have also been having slight pain in my left side. i was just wondering what this could be?