Is my dog food giving my dogs diarrhea?

I have 3 pit bulls (age 1yr) and I think their food is giving them diarrhea. They WERE on Purina Kibbles and Chunks and were doing fine, but I wanted to get them a better dog food. So, I switched them all (gradually) to Nature's Recipe Lamb Meal and Rice, which is supposed to be better for them. But now they all have diarrhea on a pretty regular basis. I need to know if I should try switching them back to Purina (which I would rather not do) or if there's a quality dog food available that is better for dogs of their size. I think it may be too much protein or something. HELP!!!!!