I am sorry to hear of Zoe's problem. Does she go outside at all? Have you taken her stool sample in to your vet to be checked for parasites microscopically? If not then it is a good idea to do this as parasites are one of the number one problems that I see. Even if she is currently an indoor cat, the parasites could have been present since a kitten and only now causing a problem.
Other things I tend to think about could be diet related, inflammatory bowel disease, infections and bacterial imbalances.
I usually suggest higher fiber food for patients with these issues and also make sure she gets checked out ( and her stool) by your vet as these situations can get out of hand.
I often give my own cat as an example. He is an indoor cat who used to be a stray who suddenly developed diarrhea. He had it for 3 years!! before we got it under control. It turns out he had something called bartonella ( a blood infection that can cause various inflammatory conditions in the body). He took a certain type of antibiotic for a month and is now perfect thankfully but he does stay on a prescription diet.
I hope she feels better soon
