I Woke Up With A Red, Bumpy Rash On The

I woke up with a red, bumpy rash on the inside of my arm. When I woke up my arm was bent and the rash is in two spots. The spots were touch when my arm is bent. This morning I put cortisone cream on it and it seems as if it has gotten worse. I took a Benadryl and put more cream on it and it is very warm. What is this from?