during intercourse is something that is very common, but should be looked into. Basically I think of vaginal bleeding after intercourse as coming from one of three places, the vagina, the cervix, or the uterus. Uterine bleeding occurs during the active part of your period. Blood coming from the uterus may get worse during intercourse, but it is not usually caused by intercourse. Thus if your bleeding is occurring only during intercourse during times when you were otherwise not bleeding at all, then I think uterine bleeding is unlikely. Vaginal bleeding is a possibility, though bleeding from the vaginal walls is usually associated with pain. I think the most likely source of your bleeding is the cervix. The cervix is rich in blood vessels and any injury
to it can cause bleeding noticed after intercourse. If your cervix contains a lesion such as some abnormal cells, then it could bleed much more easily.
For these reasons, I think you should be seen by your OBGYN very soon. He or she can perform a pelvic exam and look closely at your cervix. If you have not had a PAP smear done recently, then having this test is top priority. After the concerning things are ruled out and the cause is determined, you can ask about methods to reduce this bleeding.