So, so, very sorry for your loss!!! Please don't beat yourself up for not knowing what/how/why it happened....but rather, take some consolation in the fact that you gave her a very, very good life! She was loved and she didn't suffer long. I know very well the feeling of loss and how LOST I feel when it happens....because as tough and resiliant as chickens can be, they can be equally as delicate, and their absence just hurts.
Big Hugs to you, and my sincere condolences,
Empathy