I think you need to feel the area I marked on your photo. That is not normal, even on a BB… however her legs look good.
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if that area feels, soft in any way. Like fat, water or air squishy. And with what
@casportpony said about the poo and dehydration.
I am thinking it looks like she might be shutting down.
I would give her no feed for a day or two, lots of water and some electrolytes. Also provide grit.
Give her a low roost, keep her off grass or hay. If you let her out keep her on a hard surface, rocks or gravel would be best.
This sounds more like a pet turkey than one of a flock. So exercise her lightly.
but please be aware, she may not recover. Do everything you can. Including preparing yourself.
good luck. Best too you.