What Causes Bleeding After Intercourse?

Hello. Me and my girlfriend recently had intercourse. She bled and is still bleeding after 4 days. The condom broke during intercourse but I had already finished. I already had unprotected sex with previous partners and my girlfriend thinks I may have caught an STI which may have triggered the bleeding. My girlfriend found the end of the broken condom in the tissue when she went to the toilet which was in her all night. Any advice on what may be the cause of the bleeding? Thanks.