Lately When I Have Sex With My Girlfriend I Notice

Lately when I have sex with my girlfriend I notice that the tip of my penis becomes red,mostly under the head of the penis. Often it goes away by the end of the day. I've been tested twice for all STD's and I do have herpes. I have never had an outbreak on the tip of my penis before. My doctor says it's trauma from sex, again this has never happened before and I'm not doing anything unusual to cause this. I did visit my doctor yesterday. The redness looks slightly raise.