Have you provided her with a litter tray? At 15 she may be feeling her age a bit and may feel vulnerable outside if she is trying to wee or pooh especially if there are other cats close by. You may need to experiment with different litters and perhaps it may be a good idea to get her a covered tray. At her age she may need to go a bit more so it makes sense to proide an indoor litter tray for her.
