Hi and welcome to Healthcaremagic forum. I understand your concerns. Bleeding during ejaculation is not common sign especially in younger people and usually it indicates certain issues with prostate, testicle or urethra. Most commonly it is related to STDs so you should do test first especially if there was history of STD as you say. Second, it may indicate prostate issues such as hyperplasia or urethritis and prostate ultrasound and PSA check should be done. Third, it may be seen in ureteral stones or uroinfection. At this point, you should drink plenty of fluids to promote urination and if this repeats, you need to do tests to verify diagnoses that I have mentioned above. I hope I have answered your query. Kindly regards. Wish you a good health.