If her stool was loose before you changed her diet, then it is likely not related to the diet change. Is your dog on a monthly parasite preventative? I'm going to assume that she is and that she is not displaying any other signs of illness. If she is excessively gassy, try adding one scoop of plain yogurt with active cultures to her meal once a day for 3 days. If she gets table food, discontinue that practice. If she get biscuits or treats, stop them for a few days and see if that helps. Soft stool can be caused by MANY different things. If these diet manipulations do not help, she should be examined by you veterinarian. Be sure to bring a stool sample with you.