Hi there ~
Some amount of bleeding after menstruation
during the course of having intercourse is not uncommon. If it is to the point where it is uncontrollable you need to see a doctor. If you have just begun your sexual relationship it will get better over time. Just make sure you are not having intercourse around the time that your girlfriend should be menstruating. I hope this helps.
Take care and have a lovely day, and keep using protection !