What Causes Hexagon Shaped Itchy Rash On Arm?

Hello. I have a hexagon rash on my right arm. It started out small and then has doubled in size. I went to my Primary care physician and she prescribed a cream ointment. The rash s center is now smooth. The outer edges of the heacagon shape have started having new small blisters around the edges. The second layer is dry and flaky with the healing. It started about is about a week and a half ago. I thought I came in contact with oils from the leaves of Tree of Heaven Stinky sumac) while pulling a few volunteer starts up. It is rarther itchy. Subconsciously I find myself feeling itchy all over.