My dog had a false pregnancy once, it made her mothering instincts go into high gear. She "adopted" a squeaky rubber duck and carried it everywhere for weeks. She made a nest for her "baby", spent hours grooming it, and if someone squeezed it, making it squeak, she came running to make sure it was okay. Once her body got its hormones sorted out, she lost her protectiveness for the duck and used it as a chew toy. I don't know if it's a similar process in cats, but I know it's not that uncommon in dogs. I've heard that some dogs go as far as producing milk.