Hello,
My first question would be what kind of food is she currently eating? I suggest a high quality kitten food mixed with a high quality adult food such as Iams.
Iams is more digestible and causes less stomach upset. Fequently changing brands of food is a no-no as well. Cat's have very delicate digestive tracts and any change causes problems.
My next thought would be an imbalance in the intestinal flora. Find a store near you that carries Culturelle and give your cat 1/2 to 1 capsule in a SMALL amount of wet food or tuna daily for 5 to 7 days.
Culturelle can be found at Walgreens or you can visit their website at
http://www.culturelle.com to find more places near you.
Culturelle is a probiotic that replenishes the good flora in the intestinal tract.
Also, have the stool checked at a vet. There could be parasites such as roundworms or coccidia. In this case, a dewormer (don't buy over the counter, their worthless) or antibiotics are called for.
I hope this helps!
Regards,