Hi thanks for your question.
The commonest causes of postcoital bleeding are vaginismis and cervicitis
that is the infection of vagina and cervix
.Vaginismis and cervicitis could be due STD
or a non STD infection.Rough sex could also be a reason for bleeding after sex.I would suggest you to consult a gynaecologist who may examine,and investigate you for cause of bleeding after sex.
Hope this answers your question.