What Causes A Blotchy Rash On The Arms And Neck?

Hi, I was really itchy this morning around my neck and arms. I thought it was my sweater, but the itchiness got worse. When I took off my sweater, I noticed blotchy rash all over my arms and neck and on my abdomen. I don't think the sweater caused it because I had a t-shirt under which protected my abdoment. I took 2 benadryl, but not sure if this could be something serious.