Phrick, it can be caused from stimulating the cervix. There are alot of tiny cappilaries there and if he makes contact with the cervix it can cause this. Also, sometimes chemestry...his semen and the cervix, but mostly from actual contact with it. It happenes alot more during pregnanacy in women who have never had the issue, because the uterus displaces the cervix and reduces the length of the vagina. But if your man is reaching the cervix it can cause that.
If you have been checked for STDs , etc, then I would not worry. If it has always been that way, probably nothing to worry about. But if it started suddenly or with pain or some funky discharge, I would make an OB/GYN appt.