How To Treat A Sore Hanging Mole Under Armpit?

I have a hanging mole under armpit and it has turned sore. It started off as a normal mole and last night it was sore and like now it has a burning feeling. I guess because of my arm is it down and my clothes rub against it if I raise my arm. It was just a small mole and it has grown just a little. I thought maybe it was getting ready to fall off, but I is sore.