There is a chance she is trying to get attention. Try moving her litter box, changing her litter, changing the type of litterbox (going from short to tall, covered to uncovered). If she had a clean bill of health from the vet, that's good because she is not sick. But it means that it is likely something behavioral. I really suggest changing the litter or the litter-box first.
From Jackson Galaxy (of my cat from hell fame)
When you see your cat going not inside the box… but getting the perimeters around it, it indicates sort of a dance around litter that is indicating discomfort. Not just physical discomfort, but spacial discomfort, that “I don’t fit here” feeling.
This was a post about a cat peeing outside the box, but if it is behavioral, I feel it may be a similar thing.
Another interesting possibility is suggested by CatFancy
Other reasons a cat may fail to use its litterbox have to do with the litterbox itself. The litterbox might be too small, not stocked with a litter your cat likes, or not clean enough. It might also be in a bad location, such as a busy hallway, hidden behind a rumbling clothes drier, next to the cat's food (who wants to urinate next to their food?), too far away, in a difficult place to reach, or past a scary dog.
"And let's not forget about the dark," said Sharon Crowell-Davis, DVM, director of the Animal Behavior Service of the University of Georgia College of Veterinary Medicine Behavior Service in Athens, Ga. "I have seen cats who used their litterbox in the day, but eliminated in other places at night. With one of these cats, it turned out the litterbox was in a closet in the basement and the cat had to negotiate stairs and make it clear through the basement in pitch dark to get to the litterbox. Yes, cats can see well in dim light, but they can't see in total darkness. Adding a night-light solved that problem."
That is one reason that your pillow may seem like a good idea to her. So the first thing I would suggest you try is to change the location of the litterbox (if you dare perhaps even in your bedroom) and put up a night light. Older cats have a hard time seeing and moving and these few changes may make a world of difference for you and your cat.