Suggest Remedies For A Painful Rash On The Buttocks Of A Child

My 2 yr old had a virus about 3 weeks ago and during that tim she got a pimply rash on her butt, she is better now but still has the rash I put diaper cream the first week and then the doc told me to use cortizone on it. Its getting worse and she says ouch when I change her diaper. She only wears diapers occassionally cause sh is potty training and I let her run around with just a t shirt when at home. please help...thank you