If a cat, especially an older cat, starts to poop or pee all over the house, or just places other than the litter box, this almost always means that something is wrong, health-wise. Cats will stop using their litter box when they have health issues, or if they're emotionally stressed out. If you can rule out any changes in the cat's environment or daily routine, like a new pet or new person moving in that the cat might be upset about, then it's time to have a veterinarian examine the cat. Older cats are often prone to health problems that can cause the cat to stop using its litter box, and a vet will need to diagnose those problems before they get so bad that they threaten the cat's life.
Maybe you should try giving her a litter box again... She might be having a bit of incontinence.
Maybe she's just marking her territory??lol well have you tried putting her old poo in her litter tray?? Is it in the corner of a room?