While I'm not a doc and can't diagnose, when a rash starts small and gets bigger and more red, it could be a number of things. Steering clear of antihistamines and cortisone might limit your options, but there are natural remedies like aloe vera or oatmeal baths that can soothe irritated skin. Still, the best bet is to get it checked out by a professional who can give you tailored advice. Stay hydrated and keep the area clean and dry in the meantime!