Antibiotics do lower immunity as you know. Staph is very common and naturally lives in the natural flora on the skin surface if everybody, unfortunately it is an opportunist bacteria that will turn bad if it gets the opportunity to do so. Vegas is immunosuppressed and it has unfortunately taken a hold. I doubt Ammoxicillin will work, different antibiotics work for different things. Ammoxicillin is known for ear and chest infections. You have caught this quickly I hope it clears up fast too.