I'm not exactly sure what this may be....

I have had protected sex with 2 women in the past 2 months, and I know protected is not 100%, but I seem to have developed a small dime size rash where my penis and scrotum meet, or more precise in the skin fold. I noticed it when I showered at night when the water made it burn a little a first. It was only a little itchy, no pain or blisters after that. I went to the doctor on the 2nd or 3rd day and got all STD testing done, with a culture swab for herpes of the area (test not back yet). The PA said it looked like dry skin that was irritated and not to really worry about herpes. However, the day I noticed it I was playing basket ball for an hour then went swimming in the community pool for an hour, where there are literally hundreds of little kids a day, I proceeded to stay in my wet trunks and go bike riding for about 2 hours, and then went running for another hour, all while in my wet trunks. Am I over worried about an STD? or can a dirty pool along with me staying in my wet trunks for the remainder of the day a possible cause for this little rash? Heat? Friction? chemicals?  Also, the rash went away by the 4th day and really only look slightly red then.  Thanks doc.