Wash it with soap and water twice a day and dry it thoroughly. That rash is the same thing as diaper rash in infants - it's fairly common among men who have not been circumcised and who don't make sure that protuberance is kept clean and dry.
Either your piss burned you, or you have the herps. So if it keeps coming back, and you've had unprotected sex, I suggest you see a doctor. Stay clean and see if it comes back if you want to make sure you really need to be checked...