Ah I see what you mean, doesn't sound like she's kicking it out then. One of our cats is 17 now and the older she gets the more fussy she is about her litterbox - I would maybe change the type of litter or even get her another one (in case she doesn't want to poop and pee in the same box like our grand ol' girl ).

Given her age (great age for a cat btw!) I'd probably get her checked out by your vet too, just in case.