Hi, I Have Had A Rash On The Backside Of

Hi, I have had a rash on the backside of my arms for about a week and a half. It started out small with a few red bumps, it quickly grew to a large area on both of my arms. It is very itchy and the original area where it showed up is now turning brown and rough. I do not have it anywhere else on my body and no one else in my family has it. I first thought it was a drug allergy, then a contact rash from a pesticide I used indoors and then cleaned up afterwards, I also thought about shingles even though I am only 42, and my last thought was it could be a rash similar to what my daughter had when she was first diagnosed with diabetes which I was just recently diagnosed and put on medication for it.