How Should Legs Rashes With Itching And Pain Be Treated?

I have a rather annoying rash between my legs at first I thought it was a simple heat rash since I get them every summer. however this time its not going away with my usual treatment. I have been treating it with Goldbond Medicated body lotion and Medicated body powder. The itch is maddening and if I scratch or even rub to try and sooth the itch its extremely painful. I have been using the powder and lotion for three days now and there is no improvement. I have to reapply the lotion and powder at least 4 times a day to just keep the itch and pain under control what could be causing this problem? I know its not an STD since I have had tests done since the last time I had sex between 8 and 10 months ago and the tests were clean.