What causes large reddish boil on armpit?
You seem to have furuncle which is a hair follicle infection. YOu need an oral course of antibiotics like Amoxycillin to completely treat it. Staphylococcal skin infections spread easily to other areas hence require oral course. I suggest you to visit a RN or any clinic for a prescription of the same. Hot compresses are ok but not the complete treatment choice.
Let me know if you have other queries.