It's very possible, Angel. If your cat has not been spayed, and she's been outside, it's very possible that your cat is pregnant.
In addition to her growing belly, sleeping more, and increased appetite, here are some signs and things to note:
Have her heat cycles stopped? Are her nipples pink or enlarged?
There are some conditions and illnesses that might also cause a swollen appearance to the belly as well. In any case, she should be checked by a veterinarian.
Here's a site outlining cat pregnancy signs and offers a ton of information on all aspects of feline pregnancy. I hope that helps!