Red itchy rash could be my middle name--I am no stranger to it. Most often it is a common fungal infection, especially in folds of skin, like the armpit. If I were you, I'd do two things simultaneously. I'd get an OTC anti-fungal skin cream and also make an appointment with PCP or dermatologist (even urgent care). I'd start off with cream and see if it works and if it doesn't or you have on-going concerns go see a doctor, who at least can prescribe a stronger ant-fungal if indicated. Summer humidity can make it worse. I do understand your post BC concern, though, and wouldn't ignore it.