It's called hematospermia. It's evidently quite common and normal... but if it continues for a long time, you bleed at other times, or it's excessive, see your doctor.
I had the same thing happen to me at one point. When I went to the doctor I was told that nothing was wrong as long as this wasn't a chronic problem. It seems that sometimes things get mixed in with semen (for example: sometimes semen is more yellow than normal, but this only means there's urine in it). If you get blood in your semen the majority of the time I would recommend you get this checked out. Otherwise, I doubt that anything is wrong.