Hiya, I know it's a little different but my Collie had a similar problem for a good few months, he ate fine, seemed himself, but was runny. We spent lots of time and money at the Vet! They tried to make me put him on expensive hypoallergenic food, but also suggested gluten-free food first to see if it was an intolerance.
Turned out he was gluten intolerant. This is apparently quite common! So i don't know if it applies to cats, but it might do. It's worth trying eliminating certain foods for a period of time to note how they affect her.
Jack eats gluten-free dry food and has gluten-free treats. I was worried about the cost, but it doesn't work out much more expensive, and he is healthy.
Hope she is OK, and that went some way to help! Let us know how she does.
Lisa x x