red rash

Hi any idea what can be causing this hives like rash? No foods have been changed his only 14weeks old and is on the same diet from the breeder. He did get the cat flu that his now recovered from then his broken out in these hive like rash the vet has giving him some antihistamine and...

Noticed this on our Luna. At first I thought it was a food stain at quick glance. She is not in pain nor does she pick, scratch or lick it. It’s allergy season here in Phoenix AZ and scorpions are also starting to appear. Should I be worried? Thank you!