Wow, that makes complete sense. Sometimes they will react better if you check them at night with a headlamp when they are on the roost.
Totally not your fault, they are programmed to act like nothing is wrong since they are flock animals.
It looks like a prolapse. Usually there is a stuck egg that causes it, but there was probably other stuff going on, or she had it for a while and she was hiding it from you if she died so quickly...