I Have Developed A Rash Under My Left Arm Pit

I have developed a rash under my left arm pit to my left pectorial. At first I thought it was a spider bite multiple times. The pain or discomfort starts very near the upper pit of my arm pit, where the arm and torso meet. I have not developed blisters, I have minor soreness where the rash is. I can function at work. It seems to be growing and is now starting up on my right side under my right arm pit and right pectorial. I am scheduled to see my MD tomorrow morning.