Go to a doctor , the prostate provides the fluids that carry the sperm and tesres make the sperm, so you might have something serious with either your prostate gland or testes, get it checked immediately.........
It could be from tearing the small blood vesels in your urinary tract, but anytime you see blood in any bodily fluid it warrants a visit to a doctor.
That's a question you need to ask your DOCTOR.Good luck.