We are talking about a topical rash around your private parts right?
Might be from critters in the chamois of your bike shorts. I don't use any creams. I just machine wash my shorts twice. First time with a little laundry detergent and some color safe bleach such as Oxy Clean or Chlorox II. The second time I run it with nothing but water. The bleach will kill most bacteria and other critters in your cycling shorts. Hang in front of a box fan and they'll be dry in time for the next ride.
Regular chlorine bleach will be even more effective at sanitizing the shorts, but it also may be harmful to the life of your shorts and their color.