Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that Maddy is so uncomfortable; she must be miserable with all the scratching and biting that she's doing.
Benadryl (as well as other antihistamines) can be given to dogs when they're itchy but if the itch level is really high, then it may not be quite strong enough to be effective. But, it's certainly worth a try. The dose I'd use for a dog this size would be 1/2 of a 25 mg tablet twice a day.
Sedation can often be a side effect for many dogs.
I'd also consider giving her a soothing oatmeal shampoo bath; let the shampoo sit on her body for 10 minutes or so before washing it off.
Fish oil supplements may help for the long term since they have anti-inflammatory effects; unfortunately, they often take several weeks to be effective so wouldn't provide immediate relief but might down the road a bit.
Allergy season has been pretty rough for a lot of dogs this year so it's possible that this is what's triggering her behavior. Many patients will require stronger drugs than anti-histamines but it may help to provide at least some relief.
I hope this helps. Deb