Bleeding or spotting after sex is called post coital bleeding. It is most commonly seen in vaginal infection like Chlamydia, gonorrhea, vaginal inflammation etc. it could also be due to disease of endometrium that is lining of uterus, polyps (mass) in uterus and cervix, fibroid tumors etc. You should consult an expert nearby for internal examinations.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I had bleeding after sex once and it lasted about 2 to 3 days and my stomach was cramping also I wasn't sure why this happen but I was scared I still to this day don't no why but someone said the man could knock it on idk if that's true tho..sorry if i wasn't much help just wanted u to know u not the only one it happen to .