Hi, this may or may not be true w/ your cat, but we had a cat that became a
diabetic because of the steroid drug she was put on for some colon issues. I know that sometimes vets will use this drug to treat cancer in our cats, too. So, if your cat has been given a steroid drug, either by injection or pill, then this could very well be the cause of her diabetes. And, as soon as this drug is stopped, so will the diabetes.
Best wishes to you. I know first hand how tough your situation is.