If the cat is physically able to have kittens and if she's old enough, then there is a possibility that she is pregnant.
When my cat got pregnant, she also started getting bigger and bigger, but since we didn't go to a vet about it, we weren't really sure until she got HUGE.
Just in case, you might want to start getting ready for the kittens though...buy her a nice, soft, big bed...and you should keep her inside just for now.