Hi, I'm not really qualified to make a guess, but would it be possible to get a second opinion from another vet? Did you actually see her chewing at herself? And, I wonder if she was fed different food when she was at the training? If so and it cleared up when she was eating something different, maybe it's food allergy related. Even if she's always been fine with her food, the manufacturer can change something and cause a problem for your dog. If she was eating the same food, is it possible some cleaning product in your house is causing irritation? There are so many things that could end up with skin/fur problems, it seems like the vet could do some more investigating before just giving antibiotics. (Sorry, throwing antibiotics at a problem without knowing the cause is my PET PEEVE!
) Good luck, hope you find a solution