Skin rash

I keep getting a rash mainly around the places where the skin rubs together.Armpits,back of the knees, and the top of the forarms,the doctor give me timodine which cleared it up but said i can't use that as a long term solution.I've tried a differant washing powder and bought some new bedding so i'll see if that helps.Could certain types of food help?i'm not over weight by any means