Condolences, Carl. Cats are funny like that; it's not uncommon for them to wander off to be alone in a quiet place to pass on. Nina is 14 years old this year, and though she's had some rough spots (namely, a long-term bladder infection and pancreatits, both conditions that we have since gotten under control), I can't keep the thought from my mind that any day could be her last. She actually has a heart murmur that was diagnosed when she was a few years old, and by all rights, she could have been gone years ago - the life expectancy for kitties with murmurs is usually less than 6 years. And with having to have her brother put down last October - god, has it already been almost a year? It still feels so recent - it's a very depressing though that she her time may be limited.
But, when she goes, we have two wonderful young kitties that will help soften the blow, though it's still going to hurt like hell. She's been around for literally half of my life, and she and her brother were born on the same day that my father died; it's an odd feeling, and I'm not entirely sure why.