hello dear, a lot of factors might be responsible for your current experience, however the possibility of you having a testicular torsion can not be ruled out since its post coital and thus the need for you to report to a nearby physician for proper evaluation and prevention of irreversible damage that may ensue from prolong poor blood supply to your testicles. however I will like to know if you experiencing fever, chills, discharge from your penis or groin swelling, as these might be pointer to STI epidymitis that you are suspecting and thus need for antibiotics. if the problem is STI, best advice would be for you to abstain from sexual intercourse for now and get yourself and your partner treated appropriately or at worse use barrier method of contraception. however if it s a torsion that does not resolve there might be need for surgery called orchidoplexy . the good news is either way u can enjoy your sexual life once treated properly and not in pain. Hope am able to help with this little info and feel free to contact me for further clarifications. thanks