Feed your girl 2-3 days/food kind. A little diahrea is not considered to be allergy but just she need some time to adjust. Even though you feed her apple and then poo 4 times, you can still feed her in the second day, if she still poo so frequent, then stop and try again one or two weeks later.
If she poos normally to a kind of food, then continutes feed her for 3 consecutive days.