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Thank you for using Just Answer. My name is Rebecca; I am a veterinarian. Please give me a moment to type my answer.
I am sorry to hear Isabel is vomiting. How long has she been vomiting loose yellow foam? Is it every day?
There are dozens of reasons cats vomit. Since the vomit is yellow, that indicates she is vomiting when her stomach is empty. Is she eating normally? Any weight loss?
This could be from liver disease, kidney disease, pancreatitis, inflammatory bowel disease, infection. Even heartworm and heart disease will make a cat vomit.The place I start is with a physical exam, by a veterinarian.You can try giving her 1/4th of a 10 mg famotidine (Pepcid AC, over the counter) twice daily to see if that helps.
How is Isabel doing? Please let me know if my answer was helpful and any questions you may have.