Don't "grab" the neck. The cat is on the floor, so just gently scruff the neck without lifting the cat up. In a few seconds release, and watch the scruff. If it stays tented up, then grab your kitty and rush her to the vet, for she needs fluids badly. If the skin goes back down in seconds she is okay. I would give your kitty pedialyte to help replace the fluids she has lost- just mix a little in with her regular water-
She could be lying in the litterpan because she is feverish and the litter is cool against her skin. She could find comfort in lying in something where the sides are pressed against her. Have you taken her temperature recently?
When you start a kitty on new food, you do so gradually, mixing the new with the old food little bits at a time, so they don't get diarrhea.
I know a lady down the way who takes care of a cat, and this poor kitty, this woman switches food on this cat every other week.
She is an elderly woman and no matter who tries to help her by telling her gently that this is a bad idea, she just keeps feeding Marsh whatever is cheapest at the store. She is drowning in vet bills because the cat is constantly dehydrated, but she loves Marsh so much- it's just misguided love, and the fact that she has so little money. I finally started buying her food for her and every week now for the past few months, Marsh is getting the same food. He is looking much better-