Do you know which ingredient is causing the reaction? I'm wondering because both foods you mentioned have the main ingredient beef. She could have an allergy to beef, in which case switching to another beef food would not help the problem. Also, I'm not positive but I don't think diamond naturals is grain free, I know there are some recipes in diamonds "naturals" line that are not grain free, but not sure about the formula you're talking about. Which could be a step down from the food you're feeding currently if she has reactions to grains (which most do).
Also, if you decide to switch to any diamond product be careful and watch out for the recalls! I know many people swear by diamond and TOTW, and I personally saw success with TOTW, but they do have a long list of recalls on their foods.
GOODLUCK! Hope your baby gets to feelin better.