Dogs and people can develop sudden allergies to stuff they've been fine with all along. My dogsitter said that her vet told her that Benadryl isn't that effective for dogs, and the more popular antihistamine now is Reactine. I would strongly suggest a blood allergy panel to get an idea of what your dog is reacting to. I did one on my Bulldog, and was worth the $500. I'll bet that is cheaper in the US. Here's her panel: