I would call the vet and ask how much Benadryl your dog can have per day, according to her weight, and if there is anything in her medical history indicating she shouldn't take it. If the vet gives you the go-ahead, give it a try for at least two weeks to see results.
Allergies in dogs, just like in people, can develop over time or in a manner that seems "sudden". Usually it isn't as sudden as people think, it just hasn't created extremely obvious symptoms for some time. Same as in humans. I didn't think I had allergies because I don't sneeze or have itchy eyes, but it turns out that my frequent ear infections were caused by allergy-induced congestion in my ears that was always getting infected. Now dogs don't usually have sneezing-type symptoms related to allergies, either (usually it is skin-related), so it could be that Brandy has had allergies for a while and only now is having a severe reaction.
My vet recommends Benadryl for dogs with allergies. You can even buy the generic brand as long as the active ingredient is the same.
Wouldn't hurt to try this while you're waiting for her next appoinment.
Bless you for being persistant in trying to help Brandy