My 3 year old has a huge rash

It is on her butt and upper half of her legs...I took her to the drs about 4 months ago, and they gave me a cream and said it would get rid of it and i keep using it on her it would stay away...well that isnt true. It went away..and came back really bad. It isnt like a usual rash...its red dots with white spots in the middle. I know they arent chicken pox...what else would it be? Should I be worried, she has a drs appointment tomorrow.

It's of course very hard to say what it is without actually seeing a picture but ask about impetigo.