It could definitely be the purina.
My girl has LOADS of food allergies, and when I switched her to raw, it all disappeared. All grains (aside from barley, but she doesn't NEED it in her diet, so I don't feed it), a few common kibble preservatives, and dyes. She was red, itchy and bald in places before we changed her diet. She's also allergic to grass, so occasionally we need to pop a benadryl. Depending on which food you go with, raw isn't really all that much more expensive. I can feed my 3 dogs (55lbs, 15 klbs, and 6lbs) on $70/month. Which is far less than we were paying for Orijen a month. Of course, I shop sales and stock up-- makes a difference.