My gerbil looks very bloated/swollen abdomen?

About 2 weeks ago, one of my gerbils began getting a little bit larger. After about a week, she was noticably larger and her tummy was starting to appear very bloated. I left on vacation for a week and my older brother watched my gerbils while I was away. When I returned yesterday, I found that my gerbil had become twice as large as she was before I left. I know that my brother did not overfeed her because her sister is not large, she is at a very healthy weight. My brother did tell me that Celia (the one in concern) seemed to rush to the food when he fed them daily, but it didn't seem like she was overeating. Although I do have a male gerbil, he has never been near the females, so I am putting it out of the question that she is pregnant. I have had Celia for about a year and a half. She is acting normal and healthy besides looking extrememly bloated. She does squeak if handled since her belly got this way, and she feels as if she could just pop. I am calling the vet today as well, but I was just wondering if anyone here had any experience with a similar situation.