I'm so sorry you lost her like that
but she was very sick and she would have died soon regardless. Her belly was incredibly taut and swollen and I would guess that she is full of infected egg material called lash egg due to salpingitis (an infection of the oviduct) or internal laying or both. She will have been trying desperately both to poop and pass some of that lash egg material and the pressure of her straining to push it out has caused her to prolapse, which means the inner part of her body has been pushed out through her vent. The strain of it all has probably also put too much pressure on her heart and it has given out.
Sadly hens are very prone to these reproductive disorders due to the selective breeding that humans have subjected them to in order to increase egg production.

Sadly hens are very prone to these reproductive disorders due to the selective breeding that humans have subjected them to in order to increase egg production.