I did the same thing, listen to vet. had him shot a combo vaccine and rabies at the same time, he wasn't eating for 3 days and had a fever, took him back to the vet and they denied vaccine is causing this. Asking me another few hundred bucks to do all kinds of test.
a lot of vets now days only follow what they learn in school and the book.
for your case, do you use any flea medication? some fleas medication causes allergy also