Can you help regarding about my dog "pitbull"

Its been years (about 2) that he is still having drop of blood in his genital. (it usually occur when it is to hot) I have consulted the vet and didnt find anything unusual about it. In my own observation he looks good and performing well as he walk and run. As I remember it started when he got an opportunity to mate to a fellow dog lover. Several days after the intercourse.
What do you think is it?
Hope you can help me.
Thank you very much.
Ingat ( take care)