My Libby has had the SAME problem!!! I took her last week to see about the itching and to get her rabies inoculation... Well, she had a reaction to the inoculation... they treated her with Benadryl and something else (can't remember)... it was two quick shots... she wasn't in respiratory distress, but swelled up with swollen hives all over... She's a beagle and those droopy lips were just so swollen... poor thing... could NOT get relief from itching...
She was fine in a few minutes...
But about the allergies... the vet said it's been a huge problem this year. We're in western Oklahoma, but he indicated it was bad everywhere. The first thing he wanted to try (before going to a more serious solution such as steroids) was plain old benadryl... At her weight (30.2 lbs) she could have 50mg every 8 hours... that's two small 25 mg benadryl every 8 hours. I bought the generic for much less than "Benadryl" and she's been great for 2 days now! A little sleepy... but I only gave her three doses and then quit the medicine. She hasn't itched since Friday afternoon after the reaction!
Just be sure to check with the vet about the correct dosage!! or google it?
Good luck!