What happens when I see blood in my semen when I knocked during sex.


Blood in the semen means a blood vessel broke. If the amount of blood is small, such as small streak, and it goes away in a few days, then there isn't anything to worry about. If you have other symptoms, such pain when ejaculating or urinating, a fever, blood that is also in your urine, or blood that continues to show up, then you need to see your doctor.