I assume you mean menses. Did your girlfriend explain why the doctor said it was dangerous? Are you or is she of the Islamic faith?
Sex during a girls period can in fact feel much better for the woman. The issue here is that you can feel kind of dirty (in a bad way) during those few days and when you are young it can be a little embarrassing too. Your girlfriend may just feel uncomfortable with sex during her period and wants to avoid you making a move on her.
Generally it is not unhealthy nor is it dirty. Now, if your girlfriend already has a sexually transmitted infection or disease then the blood flow would increase your chances of catching it. However, if this is the case then you should always be wearing a condom. There is also a small chance of pregnancy during menses and to be on the safe side you should keep using your normal birth control.



