What Causes A Smooth Skin Rash Under The Arm?

Hi I have a large smooth rash under my arm. Am on 4th day of sliding scale of prednisone. Rash seems to be slightly larger. I have no blisters, is not painful or itchy. I have a large wooded lot and am an avid gardener. I know we have poison ivy but not sure of oak or sumac. The rash first occured about a week and half ago. Could this be shingles without the the blisters and all the other shingle symptoms?? Thanks!