We used to have a dog that had seasonal allergies with hives, every year, for about 2 weeks. We never figured out what it was, but our vet had us give 25 mg benadryl every 8 hours for a week, then stop. If the hives came back, we started again.
He was funny, I always took him into the kitchen to get his pills. One night he woke me up in the middle of the night, when I petted him I realized he was covered in hives, when I got up he ran to the kitchen and waited for me, he related his pills to his itching…and to it stopping too, I guess. Of course that night I was out of benadryl, so had to run to the all night drugstore at 3 am!
Definitely start giving benadryl and keep it up for at least a week.