I'm not a rabbit expert, but if all seemed fit and healthy when born and there didn't seem to be feeding issues, then I would suspect a disease, for them all to have died at the same time. (Unless she killed them)
However when you say they all had full tummies - could it be that after death the tummies had swollen due to bacterial fermentation? I have just remembered a ewe I used to have that had 2 healthy big, bouncing lambs. All seemed great for a day. Then on day 2 they were both dead. Turned out her udder wasn't giving any milk and they'd died of starvation. Could it have been something like that, or that the doe had developed mastitis? I know it's probably a bit late now to check, but at least you'd be able to see if she had mastitis.
I seem to recall you used to work with pigs. So what would you suspect if you found a whole litter of piglets dead?