If she was that close to delivery to be that large she would probably have dropped in the back too. Bet she has a hernia or a tumour. With those hernias they can be huge looking but basically just be filled with fluid or air. Or intestine, that would end up killing her. I have seen things like that on sheep, which are basically the size of a big doe, and it isn't pretty.

Could also be an abcess or a tumour but I would go with tumour before abcess.