rash?!

I need some help. This rash popped up under my arm about 4 inches from my arm pit. It started out at 1 bump that I scratched by accident just once. Within 2 days it had turned into these painful, red blister looking things. I've been putting aquaphor on it to try and ease the pain, but it doesn't help. has anyone had something like this or seen it? I'm trying to figure out if I need to call my obgyn or if I can just let it run its course. I'm 25+2 today.