Rashes are really hard to give advise about over the internet. It's likely that it's one of 2 things. Either ingrown hairs or something related to heat.
Try these couple things and see if it helps. Get yourself a pair of those bathing gloves you can get at walmart. They have a rough textures. It may get rid of or lessen the extra/dead skin that promotes ingrown hair.
OR..
In the case of it being heat related, always wear cotton underwear (you should do that anyway). Use powder with cornstarch in it after bathing. You can also get antibacterial cream at the pharmacy without a prescription. Another possibility is to use something meant for a babies behind.
This is a very common problem. It happens mostly to people sit alot such as students, people who work in offices etc.
Good luck
embarrassing question--please answer 
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