Since you've taken the proper steps to determine if this is just a personal preference with the litter substrate or cleanliness, it's time to get her into a vet. I'm glad to hear she's acting normally in other ways (eating, drinking, behavior) - this is promising.
In a geriatric (older) cat this could be a symptom of intestinal infection. Anything from a parasite to a virus; bacteria or fungus. A vet really needs to check her out before it goes too far, too long.
Hopefully this is nothing but a behavioral thing. Once the vet rules out physical involvement, we can look at what's going on that has made her insecure and/or passive-aggressive in behavior.
Check back with me ok?
Edited by Serenade on August 1 2006 at 5:50 PM