Blood in semen is caused by Hematospermia, and it is caused by different conditions which affect the male genitourinary. Affected areas are the bladder, urethra,and the testicles,these conditions are uncommon.
Injury to any of the organs or glands involved in the making or transfer of semen to the outside of the body can cause blood in semen. If this continues to happen I suggest that you see your doctor, because this is not normal.
