Welcome to HCM.
Most likely it is blood in the semen. In most of the cases no cause could be found for it. It may be caused by a small stone in the seminal duct or swelling of the seminal vesicle
. If it recurs you must consult an urologist
who will have to examine you, do the necessary tests, including ultrasound
study of seminal vesicles to try to find out the cause. It is not serious and don't worry. But if it recurs it will be a good idea to pay a visit to an urologist.
Wish you all the Best.