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What you are seeing is usually caused by a parasite problem or a protzoan problem. You not only have to worry about parasites but you also have to worry about coccidia and giardia. I suggest that you either see the vet for a fecal exam or take a stool sample to your vet to rule this out (or in!) before you try anything else.This could be solved with a simple deworming.
Keep in mind that parasites can cause the cat to feel terrible and therefore they will not be as active, will have poor bathroom habits and can be a general problem.
Until you can see the vet, you can switch her over to a bland diet of baby food only (no treats, table food or cat food). Give small amounts throughout the day. Be sure and choose a product that does not contain onions or garlic.
Let me know if you still have questions about this.
To save money, you can take a stool sample to your vet in a plastic bag instead of make an appointment. Most vets will not charge you for an office visit if you do this. After they take a look, they can tell you if an over the counter wormer might work or if you need a prescription.
I hope this helps more!