Go to a doctor. It could be shingles and very contagious.
I can tell you about the red rash under your armpit, b/c i used to have the same exact thing for like a week. I'm telling you, it might just be the deodorant u use and when it rubs up against your clothing it creates an irriatation rash. That used to happen to me when I put too much on and then went to the gym and sweat a little, or when I just put too much on it would rub. Try using a different kind, or using less of it.
I used to get that when i was fixen to get my period... with mine my knees arms and face would get red itchy and achy the doctors told me that it was just how i am and the way that my body reacts to hormones and such... I dont oviously know if this is the case for you aswell but i know my red rashy looking things would be very hot to the touch and the best thing that worked for me was cold compresses, ice water, and laying down infront of a fan to relax... i hope all is well!