Oh no! Sorry for your loss! I wonder is she could have had some cancer like thing? I really don't know. It's possible that the infection was a lot worse than you thought. Can you do an autopsy on her?
If she is acting normal, perky, eating, etc, than she will probably be fine. It has only been a few days since you discovered her, so she probably needs some more time to heal. I had a barred rock hen a few years ago that had a similar injury as your hen has. The wound smelled terrible and was...
Wow that looks nasty! Does she have a wound under there? In the last picture it almost looks like she has a giant scab under the poop. Taking her to the vet is probably the best thing to do. Keep us posted!