Suggest Cause For Warm Rashes On Arm And Wrist

I've been taking prednisone for over 36 hours to help an allergic reaction in the form of a pink, puffy, warm, dot-free rash that spread from the underside of my right arm, out to my wrist, down into my armpit, across my chest and stomach. I don't know what it's from. After 12 hours the prednisone seemed to help a little bit, not much. But now at 36 hours (and I haven't missed any doses), the itch is getting worse again and occupying more of my body. Should I still be reacting or do I need to go back to the urgent care?Thanks you. Erika.