Why would my adult cat have a bowel movement in her own bed and stay there?
From the Editors of The Daily Cat
It's good that you are concerned about your cat's inappropriate elimination problem, beyond just cleanup worries. A healthy cat is usually very clean. If your cat was feral, the behavior you describe could draw predators right to where the cat is most vulnerable -- sleeping in its resting spot.
Sometimes, cats will use uncovered feces to mark territory. Do you have other pets in your home, and does your cat seem anxious around them? Or have you had some other new addition that might have stressed out your cat? If the answer is yes, it could then be possible that your cat is marking territory by eliminating. This type of behavior is called "middening."
What we do know for sure is that your cat is avoiding its litter box. Since your cat is also staying in the bed where it eliminates, I don't think an unclean litter box is the real problem. You should have your cat examined by a veterinarian to determine if a health issue underlies your pet's behavior. Cats, if they feel pain when they go to the bathroom, will often associate the litter box with that pain and then avoid it in the future.