What Causes Worsening Bright, Itchy, Red Rash On Vaginal, Anal Area?

my daughter is 4 yrs old and has a bright red rash on her vagina and her anal area. its been very itchy and her doctor gave her nystatin to put on wev been using that for several days now and it does not seem to be getting better almost worse. Now she says it husts to pee