Is my chihuahua pregnant.........?

my dog (her name is Chubby) disappear one day and reappear after 30 minutes at my backyard gate. after a few weeks, her stomach became huge and her are fat. i am getting sort of confuse, because it could be that she is fat, or that she is pregnant. i compared her to her older sister (named Chewy). Chewy wasn't oversize, but chubby was. is she pregnant or what???????