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Hello, this is Dr. Jess.I am sorry to hear about your dog. Did the benadryl seem to help him? How much does he weigh and how much did you give?
This does sound as if it could be an allergic reaction to something. Sometimes benadryl alone is enough to treat this, but some dogs end up needing some steroids as well. The safest thing to do in these cases is to visit your vet. If the reaction worsens, it can become life threatening. If the swelling increases,he develops hives elsewhere on his body, he is vomiting or lethargic, or ANY difficulty breathing develops, then I would recommend taking him in to the vet immediately.
Here is an article with info on benadryl and dosing: http://www.petplace.com/article/drug-library/library/over-the-counter/diphenhydramine-benadryl