Making this harder is the phenomenon of phantom pregnancies where a cat will often show some of the signs of pregnancy without actually being with kittens. (healthguidance.org)
According to PetMD, phantom pregnancy in dogs, also known as false pregnancy or pseudopregnancy, is most likely caused by hormonal imbalances, especially of progesterone and prolactin. (reference.com)
Cats have a different type of reproductive cycle and therefore the conditions under which phantom pregnancies occur are different. (petpeoplesplace.com)