What Causes A Ring On The Head Of Penis That Swells And Itches Post Intercourse?

I have something that looks like a ringworm at the tip of my penis.Went to see a doc but she did tests and still couldn't find whart was wrong or the cause.I was told to buy a bactroban cream.It swells a bit after sex nd gets painful wen I get an erection.What could be wrong with me and what is the cause?