It sounds like hives. The severity and persistance of hives will vary from dog to dog, and what caused them.
It is important to be aware that a reaction which causes hives can become life threatening. Hives are an external swelling, the same thing can be happening internally, and it is possible for the swelling to close the airway, causing suffocation.
I'd speak to my vet. There are stronger medicines which may be necessary to get hives under control. Rocky used to have severe reactions to ground mold in the late summer and early fall. It was a contact allergen, and affected the tissues between his toes, and on the inside of his ears.
Benedry worked fine to control "average" reactions, but the vet gave us prednisone (I think) for the times that the benedryl just didn't cut it.
He'd take a series of a few prednisone, then a maintenance dosage of benedryl gave him relief.
There is another weapon you might want to consider having on hand. Ask your vet about an epi-pen. These have saved many pets lives who were having severe reactions.
If your dog is having hives, examine its lips and gums. They should be pink. If an animal isn't getting sufficient oxygen, they will lose their pink color. An epi-pen will counteract the reaction much quicker than an oral dose of benedryl or something stronger like prednisone.
While life threatening reactions may not be common, if you have a dog who is sensitive to various allergens, it is wise to be prepared. Your vet is in the best position to evaluate your pup. They can provide you with the appropriate meds, and instruct you on how and when to use them.
Tom
