Diarrhea in kittens can be very frustrating as there are so many potential causes. But in your case, it may be because you are switching foods too quickly. Feed her a high protein, low carb canned cat food (not fish). For kittens, I like to start them on canned kitten food (chicken). Iams makes a good canned chicken food for kittens. Add a little Hill's W/D to it for fiber. (Pumpkin is fine, it's just that cats don't like it).
You'll need to stabilize her digestive tract before you do anything else. She may also have worms.
Use a good broad spectrum dewormer such as fenbendazole (Panacur) as it also gets Giardia. If there is blood in the stools, the culprit may be coccidia and a sulfa drug (such as TMS) must be used.
Responding in a prompt manner is the key when it comes to fighting diarrhea in cats and kittens. Some of the measures you can take at home while waiting for the vet appointment are:
Feeding plain yogurt with active cultures.
Cooked hamburger and twice-cooked rice mixed together.
Bene-Bac powder- available at any good pet supply store.
GSE (Grapefruit Seed Extract) Liquid- 3 drops in wet food.
Adding fiber to the diet to see if the stools become formed.
Kitten can become dehydrated quickly, so check for dehydration:
While the kitten/cat is at rest, lightly pinch the scruff of the neck, pull up gently, keeping the cat firmly on the ground.
Hold the scruff for just a few seconds then release.
The scruff should fold down to normal in seconds, if the scruff/skin stays tented up, get your cat to the vet immediately.
You can also check the gums by lightly pressing your finger against the side of the cat’s gums. The gum should go white and then pink up quickly.
If you see streaks of white on your cat’s gums or the gums are pure white or yellow, get the cat to the vet quickly.
The third eyelid (called the yaw) will also show when a cat has become dehydrated.
What NOT to do:
Do NOT give over-the-counter medications to your cat, unless the vet okays it.
Do NOT deprive your cat of food over 24 hours. You can withdraw food for one day to rest the gut, but any longer your cat stands a chance of developing a worse problem than diarrhea.
Do NOT use over-the-counter wormers, or other pet-aids that you think will solve the problem.
Do not self-diagnose your cat.
Good luck.