I had that happen and it was congestive heart failure (I was told here that if you open the cavity and there's yellow liquid, it confirms CHF). I started taking away their food for 12 hours a day and didn't lose any more (well one more the next day was almost dead, but not totally, so we butchered him).
We threw out the first guy since we weren't sure what happened, but the second guy we knew for sure was CHF so we processed and ate him.