Call your vet and get her in as soon as you can. They can give her a short-term anti-inflammatory shot to relieve her itchiness until you get her on a new food. Food allergies don't normally show for several weeks after switching foods, according to my vet, so I would guess the food to be the culprit. The vet may want to try a bland or prescription diet for awhile to see if that helps, and you can look up brands of food that are good for allergy-sufferers online. If you can narrow down what ingredient may've caused it, that'll help too. The corn is always a first suspect, but even chicken could cause it. So compare labels on her previous food and current food and see what's different.
My baby suffers from environmental allergies, so we've been there done that with the food trials and itchiness.
Oh, and Benadryl may help, but it may may not and make her extremely drowsy. I would recommend the vet first. We never had very good luck with antihistamines.