Rash under pugs leg really worried any suggestions?

I have a year old pug and under its hind leg below the belly region were legs meets the hip there was a rash that started about a week ago it started off real small and has developed into a really big rash she wont stop licking it! It has developed into a bright red brownish color i took her to the vet and they said it may be food Algerians i changed her food to what they said still didn't work any suggestions medication esc.?