Red Blotches On Head Of Penis After Sex, Feel Itchy. Temporary Relief With Medication. Possible Reason?

Hi there. Every so often after sex with my girlfriend (immediately after sex) i notice some small red blotches on the head of my penis, occasionally some redness just underneath the head too. It doesn't really hurt, can occasionlly feel slightly itchy but that's it. It goes away after 5 - 10 days by itself.. then I can have sex repeatedly without it occuring, then it flares up again. Most of the time the redness is in the same area/areas. No discharge, no ulcers, no swelling. I went to a doctor and they prescribed an ointment cream which seemed to rapidly heal the issue after 3 or 4 days when it occurs, but it does seem to come back. Although the doctor at the time identified it by sight alone and said I didn't need STD testing, I opted to have a full range of STD tests anyway, (blood, urine etc) all came back negative. So what am I faced with here? Could it be bacterial and is not related in any way to the sex itself, but hte sex is just causing it to become inflamed/irratated? I can't remember the name of the ointment, but it was anti-fungal as well as either steriod or antibiotic? or anti-bacterial. Basically it was a combo thing. Could it be it wasn't strong enough to completely clear up whatever it was? This has been ongoing for a few months but only used the oiintment the last 4 or 5 times it has happened.