It shows she's almost ready to deliver. Think about carrying around a bunch of babies in your tummy that hangs down. That's heavy and a lot of work, sure she's going to be breathing differently from exertion. She's working ten times harder to walk. It's a good thing that she's eating and drinking a lot too! She's eating and drinking for several puppies and herself. If she were to stop eating and drinking, that would be a problem.
If you're still concerned, call the vet and ask