Please stay on the formula. Dilute the formula a bit with more water and be sure you aren't overfeeding. The formula might be to rich and the kitten's stomach isn't developed enough to handle that. You can also add Pedialyte to the formula. Are you feeding KMR?
Very Important! Be sure the kitty isn't becoming dehydrated. They can fade quickly if they do. Very, very gently, pinch the skin at the nape of the neck. Hold it (gently) for a second, then release. If it folds down to normal, you are okay. It it stays sticking up, get the kitty to the vet NOW!