I had my gallbladder removed 11 days ago, 3 Days after surgery I noticed a rash all over my stomach and one day later I started getting a rash on my arms. Was told by surgeon to take benadryl, that I most likely had an allergic reaction to the soap they used.  Took it for two days and was fine, but after another two days of stopping benadryl I began itching in the same places.
You haven't mentioned if you are taking any kind of medicines or not. If allergen is present, Ben Chat Doctor.