Usually this type of bleeding is from the cervix, and not from uterine hormonal/cycle bleeding.  hormonal fluctuations can cuase the cervix to be soft at certain times (during ovulation) which can make it more likely to bleed during/after sex.  Make sure your pap smears are up to date, as it may also be a sign of changes in the cervix.  other factors, such as thrush infection, can also cause that area to be more sensitve to post sex bleeding.  If it happens again, see your dr if worried.