What Causes Painful Rashes Under The Breasts And On The Back?

About a week and a half ago I started to feel sick, kind of like the flu but didn't get a fever. 3 or 4 days later I began to get a rash under my left breast, it was very itchy and burned a little as the week progressed it started to spread in a line generally speaking toward my back. There are little bumps on the rash but not big blisters. the skin is sensitive and itchy but the muscles under the rash hurt very bad. it kind of is like a stabbing pain but only right under the rash area. I want to know what you think it is and if I should go to the doctor because I don't have health insurance.