What I would do is check the food ingredients and see if there is something you find. Then go to the veterinarian to make sure she's not sick. Changing food is also very hard for their stomachs to change to, so I wouldn't be surprised if it was just the cat trying to get used to it.
If you are in an area where parvo is prevelant, those are signs. I would seek a vet as soon as you can. They will dehydrate quickly and that is why she is drinking, but quick vet attention is the only help for parvo, esp if she is under 6 months.
The kittens age could be your problem, sometimes dryfoods are indigestible for kittens. Her drinking water is a good sign, she knows to survive she must compensate the fluid volume lost from vomiting. I would suggest weaning her to the dryfood by saturating it with warmed canned milk (Pet Milk can be purchased at Walmart .49 a can) then gradually withholding the milk until she is able to digest the dryfood completely. This should only take about 4-6weeks.
If the vomit is very foul smelling, runny and bubbly and the diarrhea has clots of blood and darkened runny stools, I would highly suggest going to your local vet and purchasing a bottle of dewormer (Pyrantel Pamoate suspension). **** General rule is to worm all cats and dogs once, then repeat in 2 weeks and then continue once a month.***** It is inexpensive and a must for all cats and dogs. The dewormer comes in a small medicine bottle with a syring measuring the dosage. You simply squirt it in the mouths.
I would go back on the other food she was on right away. Changing food can be very hard on some animals that have sensitive systems. It needs to be done very slowly, like over a 10 day period mixing the food a little more each day. Beyond that, if this doesn't stop like real soon, as in maybe 2 days, please know 2 days is just my guess. I would be in my vets office right away. This can kill a kitten real fast if you let it continue. I hope everything turns out ok for you and your new little friend.
