My mouse can't move her legs?

She drags herself around and burrows under her bedding and simply relaxes there. She has a bulge in her stomach and her legs look slightly swollen, but she just drags her feet and pulls herself around her cage. She doesn't act as if she is in pain (no squeaks, no other signs of illness, etc.) Is there anything I can do to help her? She was fine until I went to feed her today. She can't be pregnant, because she's four months old and her only roommate is a female.