it could be a number of things. Speaking from a whole FAMILY of sensitive skin, it could be detergent, it could be her shampoo or body wash, it could be a heat rash, it could be eczema, etc.
I dont know if you are comfortable buying stuff online, but that is what i started doing so that i could buy body wash, shampoo, lotion, and laundry detergent, etc, from the same place and it would all be free of stuff like formaldehyde or phosphates that can causes rashes and health issues. Its a lot cheaper to buy it online then it is to have to buy the chemical free or naturally based stuff at the store.
check out http://www.HealthySafeAndClean.com there is a lot of info there on chemicals and how it can effect you, common products that have caustic chemicals, etc. you can request more info if you want too.