Bleeding during sex

For the last few months, everytime my girlfriend and I have sex, I bleed. Sometimes it's not much and other times it's like I'm on my period. There's not much pain so that's good. But it's worrying me. I talked to my doctor and he said that he'd be more concerned if I was bleeding all of the time and not just during a sexual encounter. I decided to get tested for STDs including HPV. The STDs came back negative and I'm still waiting for the HPV results.

Does anyone else have this issue? Any thoughts?