Is this a sexually transmitted disease?

So I've had a red rash on the head of my penis since the middle of march and i remember it appearing a little after having sex with this one girl but we used a condom. Its been red from the tip to the bottom of the head at the front and it hasent gotten better or worse since march. Does anyone have any idea what this is? my doctor said it probably isnt anything serious but and she perscribed me lamisil cream which hasent really helped all that much. please help