Well, if she's been on Pro-plan, then the most likely causes of the problem are the corn gluten, the wheat, the beef tallow, the brewers rice or the beet pulp (probably in that order too). That doesn't make it
impossible that something different in the food is the cause, or that it's an environmental allergy either.
But upgrading to a better food is a good idea, and very beneficial to your dog irrespective. We are what we eat - and that's true for dogs as well as people.
As a suggestion, how about one of the Fromms Four Star range of foods? Those are very good quality, and aimed somewhat at allergic dogs. Foods like Innova, Chicken Soup, and Wellness would likely be good choices too. They're not allergy formulas - but they don't contain nasty things like corn gluten and wheat either, so are far less likely to cause a problem in the first place
If you find you do need a full allergy food, then good options are Wellness' Simple Solutions range, California Natural, or Natural Balance.