Blood from Urethra

Last night my girlfriend and I had sex for a really long time. It was quite rough as well. When I started to orgasim about the third ejaculation was extremely painful. It felt great until then and I could feel the sperm come out very hard in one painful burst, after that one it felt great again except for the confusing pain of the time in the middle. When I went to the bathroom and looked I sqeezed my penis a little bit and blood came out. I had felt this before with this girl, but no where near the amount of pain and no noticable blood. I cannot remember when exactly or what position I was in when I came. I am concerned that this will get worse, if I went to the doctor does anyone have any idea what the heck he would do? Or should I just try to see if it happens again? It does not hurt when I pee nor did it hurt right after this happened. Only when I came. Just for pure information, I was behind her when I came in her. Any information would be great. Thank You.

P.S. I can feel it is a little sensitive today.