if it were me i would go in and sit there until they can squeeze you in or go to another vet as hives are not good as they can swell up
my dd has horrible allergies and she had hives once as she licked dh cologne on neck and she had welts with an hour later. HIves are from an immediate contact to something. The hives took a few days to clear and we used benedryl and temaril p to kill the cycle. I believe it is 1mg of benedryl per 1lb of dog and i used the 25mg tablet and split it and i have temaril p due to her allergies so did combo of both per the dermatologist.
It is an easy fix but you need to get doseage from vet and i was concerned about throat swelling when i was at work so i called emergency vet and dermatologist we see -- you can google it as it is canine hives