I was hoping you would get lots of replies to this. I don't know the answer, but I did spend a whole spring worrying about a ewe which had one side much bigger than the other, like a big lump. It turned out that was just her, and sheep's insides are lopsided anyway. But if this is a discrete lump then it could be she's herniated something, but without seeing the animal it's hard to judge.
I hope you get some more ideas.
How is she today?