Hi there and welcome to the forum.
If you could give some more info on your piggy hopefully we can help. Is she a lone pig, how long have you had her, what's her health history.
There could be a number of reasons why her belly is enlarged, the most obvious would be pregnancy. Bloat, ovarian cysts, abdominal mass, the list goes on. The only person that could tell you what is going on is a vet and as you are concerned enough to find and join this forum to ask I would suggest that you arrange a vet visit so that you can find the answer and set your mind at rest.
Suzy x