Hello, I am Dr. Elizabeth.
You must be wonderful owners if you have had such long lived kitties!
I suspect that Poco has fluid in her abdomen. I would guess that you are noticing that her back bone is very prominent? That is a pretty classic presentation for loss of muscle mass due to an underlying problem.
Unfortunately I would put cancer highest on my list of causes for fluid in the abdomen. I would approach Poco's case by likely first tapping some of the fluid from her belly and then looking at it myself or sending it to a laboratory for analysis. This can sometimes give us a diagnosis, but often just narrows the list of possibilities.
Poco should also have routine blood tests and a urinalysis to check kidney and liver function. You are very familiar with kidney disease and I don't suspect that in your cat as the primary cause of the fluid.
Other causes of fluid in the abdomen are feline infectious peritonitis (rare in a 19 year old cat), congestive heart failure (unlikely) and liver failure.
The stool color change may be due to lack of intestinal ability to digest fats appropriately. This is probably due to the underlying problem.
An abdominal ultrasound would be very helpful in your cat's case and if you were referred to me it would be what I would do if I could feel any masses or changes in her abdominal organs.
I understand that Poco might not be a very willing participant in diagnostics? As I said above, tapping her belly is a fairly easy procedure that might yield a lot of answers.
I do recommend seeing your veterinarian for a thorough examination of Poco. They may find something even on examination that is obvious and may help you make decisions as to how far to investigate in your old kitty.
I hope the best for you and Poco.
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