Not that I have anything valuable to offer, but we found one the same way during the extreme heat we had a few weeks ago. I went to open the coop in the morning and she was in the nest box with a bluish comb and very bloated and heavy. She had laid an egg the day before she died and seemed very healthy when I locked the girls up for the night. It was totally weird but all the other chickens have been fine. Sorry for your loss.