I'm with
@-Flash- that she died from sudden death syndrome. This can happen to any hen, not just production breeds.
The little clues you presented to us, that she'd been laying an egg every day without fail and that there was an egg in position to be laid when you did the necropsy point to SDS.
What happens is that calcium levels are low in a hen and as her egg reaches the shell gland, it draws heavily on her calcium levels and not finding enough calcium in the shell gland, her body sucks all of the calcium out of her blood and this causes a heart attack.
It's so tragic, but especially so in such a young layer.