Such a rash which appears all of a sudden can e due to hives. Wear cotton clothes and use calamine lotion. Also use mild soaps and you can take some antihistaminic lie Benadryl or Claritin. If the rash spreads then get it evaluated from a dermatologist as you may need oral steroids which should be taken under medical guidance. Also wear cotton clothes and use mild soaps like Dove.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Basically a dry skin reaction. Moisturizing cremes should help.