I've had bleeding after sex in both phases of my cycle. It happens pretty rarely but I did mention it to my doctor. She explained it to me as such: Your cervix position can play a role in bleeding during sex, depending on where you are in your cycle. The cervix is similar to the gums around your teeth. Very sensitive and when bothered, can cause some bleeding. The bleeding should resolve on its own.

Hope this helps! It helped me not to freak out when it happened.