Armpit Rash-- please help!

Hello, I'm a seventeen year old girl and just last week, my armpits broke out in this odd red, bumpy, itchy rash. At first, it wasn't anything I was worried about, i thought maybe it was razor burn. Well as the week progressed the itching started and it burned when I would put on deodorant. Now, anything I put on the rash burns intensely and its started spreading underneath my arm and to the side of my breasts. I've gotten really worried! What do you think this could be and how do you think I could treat it?