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Hello, this is Dr. Jess. I am very sorry to hear about your cat!!!
Because she is an older cat and is having increased water intake, this is a concern which I would recommend a visit to your vet for. In older cats, we do worry if they start drinking increased amounts of water, as this can be a sign of kidney disease (very common in older cats, and can also cause nausea and vomiting), diabetes, and thyroid disease. The safest thing to do would be to have your vet examine your kitty and run some bloodwork to check and see if all organ function is normal. It may be coincidental that these symptoms have arisen at the same time as you bringing home a new kitten, and I would hate for you to assume it was a behavioral/stress issue and delay getting a diagnosis and starting appropriate treatment for her. We can see inappropriate elimination (pooping or peeing outside of the box) related to stress, or decreased appetite, but it would be unusual to see vomiting and increased water intake as a result of stress or household changes.