I had oral sex with a stranger and after that I've noticed a small sore inside my lip. The first time it was there I consulted a dermatologist and she prescribed azithromycin after which the store went away. The sore came back after two weeks and it keeps going and coming back at the same spot. It is firm and not really painful. I've done hsv, hiv, syphillis tests which have all come back negative. Any suggestions on what to do next?