What Causes Raised Itchy Patches Covering Arm?

I just woke up with raised itchy patches completely covering one arm. i am a 27 year old female. 5'5", 125 lbs. i have had type one diabetes for 17 years, but it is mostly well controlled. no high blood pressure. no high cholesterol. i also haven't eaten anything out of the ordinary recently. i did stay at my sisters house for a few nights and returned last night. she has a new kitten, but i've been living with cats for years.