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Hello. Thanks for the question.
It sounds like she may be having some allergies. Benadryl can help, but she'll probably also need a round of steroids (Prednisone) to get this resolved. I would talk with your vet in the next day or two to see if they can help out. Otherwise, a topical steroid with Hydrocortisone may help here.
I would also make sure you're giving the correct Benadryl dose. The dose in dogs is 1-2 mg per lb body weight. In a big mastiff, you're likely to be giving 4-6 capsules at a time.
I hope this helps, let me know if you have any other questions.