That would seem to tip the scales toward shampoo residue.
Let us know how she does after a thorough rinsing!

It also occurs to me that once the irtching/scratching cycle commences, it sometimes continues for a bit, especially if the skin has become irritated, so another night of benadryl may be in order. There's a guideline of 1 mg./lb of dog every 8 hrs. (subject to veterinary approval) but you probably know that.
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If she's still itching after that, or if she's developed any hot spots, I'd call the vet.

Doc