What Causes Red Patch Under The Armpit?

I am a 73 year old male with type 2 diabetes . I stand about 5/9 and weigh 192 pounds. I seem to have a very red patch under my armpit that is sore to touch. Other than that I am only aware if I use deo which I havent used the last two days but have been using polysporin. So far the reslts have not been noticeable. I would appreciate your input as to how to treat this successfully.