I would bring her in, or at least call them, it is not normal for any cat to get a distended belly (unless pregnant). It can sometimes mean that they ate something they were not supposed to and has now obstructed their system.
A distended belly on a kitten or a puppy can mean the animal has worms. If she has a bowel movement you should check it for worms. I say this because worms are exceedingly common in kittens and puppies and they can get sick and become unplayful very fast if the situation isn't taken care of.