Allergies Rash?

It seems my 3 year old has allergies for the first time. Hes congested, his eyes are itchy and some days they are red and swollen. But he also has an itchy rash all over. I took him to see the pediatrician a few days ago and she wasn’t sure what the rash was - didn’t look like measles or hives and she wasn’t sure if it was related to his red swollen eyes. The rash has gotten somewhat better since. We have him on Zyrtec which seems to be helping. But he’s still itchy. Anyone else experience this?