Where exactly is the rash? Under or below the short line? If under your shorts, it could be a combination of sweat and laundry detergent causing contact dermatitis. I had the same problem. I cannot use power laundry soap, only liquid.

If below the shorts, is it possible your legs are rubbing up against something? Again it's probably conatct dermatitis - you just need to figure out what is the irritant.

Also what is the material of your shorts? Does it include cotton? Microfiber? what? This may also be the problem.