well from rubbing. you ever herd of starting a fire well i know people say that. you might not be fat but your weight is carried in your thighs and they rub and will leave you with rash.
the treatment for a rash on your innr thighs is to clen it good, never scrath, mabey rub some cocoa butter on it. if it does not go away soon i suggest you go to a doctor and try not to ge it again