Reddish Raised Bumps Spreading On The Body. On Cortisone. Allergy?

My body spontaneously broke out into a rash of red, slightly raised bumps this morning. In a matter of seconds they showed up across my lower abdomen, then spread to my thighs, rest of trunk, and upper arms. So far they have not shown up on my extremities. They do not itch, and I hadn't eaten anything before they broke out. I put cortisone on a small patch as a test in case it might be an allergic reaction, but no response positive or negative. I was going to take some benadryl to see if that would help, but given that they don't itch I didn't know if that was the right way to go. Any thoughts/insights you may have would be appreciated. Thanks!