I have not had issues with food with any of my cats - from a digestive tract perspective (TG), but have read that if there were some sort of allergic reaction or something in the food that is upsetting a cat's system, it can take a while to manifest itself, and then to resolve once the food is removed. Have you compared the ingredients in the two foods, just to see what is different? If you find differences make a note of that for the vet as well. Add to the list to talk to the vet about trying something like Pepcid for her. I don't know if that can counteract an irritant in the food, but might help if she would by chance be experiencing acid reflux.Lola was eating new food for about 2.5 weeks before these issues arose - that’s the only thing that makes me think it’s not her food. Then again I have not transitioned a kitten to adult food before, is it common to have a reaction after several weeks of the new food? It’s a good point you have made though, if that is the cause, maybe her little stomach is still recovering.