11yr old cat pooping on floor

Our 11 year old cat, Smoke, has always used her litter box. Approximately 1 1/2 years ago we had to take in her mother and our cat started to poop on the floor, but she would pee in her litter box. Her mother, Tammy, used to pee on the floor. We put them in a room together because they wouldn't get along and once we put them together they were fine. About 3 months ago Tammy had died. So we let smoke run free of the house again and she was back to normal. Until approximately 3 weeks ago she started pooping on the floor again. She occassionally poops in her box and she still uses it to pee. She only poops on the floor at night when we are sleeping and it's only once a day. We caught her doing it once before in the past and she will sniff around the floor awhile before she does it. It's never in the same spot but only on the carpet. Any suggestions?

Because of her advanced age, I would suggest a trip to the vet to ensure that it isn't being caused by a medical condition.

You might try a new brand of litter. At that age, it's a little harder for them to dig, and some litters can iritate their paws. She could be avoiding using it. She could also be having problems finding it at nighttime. What's her vision like? You might place another closer to where she sleeps, and provide lighting.