Thanks for your question and concern. Blood in semen is to be treated and investigated as soon as possible. Common possibilities of this is, minor rupture or trauma in prostate, and infection in seminal vesicles. Mostly, this will resolve by itself. But if you are seeing it for long, take immediate medical aid. Make an immediate visit to your urologist if the bleeding seems to be consistent. He can rule out a chance of tumor in prostate. Good luck.