It very well could be age... but if the litter habits have been getting gradually worse, it might be time to add easier to get in and out of pans in extra locations... of course keep the one she's used to, but even if thevet did not find artheritis, if she's going next to, it might be a getting in and out issue (or could just be she feels that she's "close enough")

Another vet visit might be in order... a urinaray infection culd come on fast and spark the behavior you described.

Hope you have more good years together.