The pooping is a good sign, and the vent doesn't seem to show signs of gleet. She could be low on nutrients, as you said the comb color improved after giving her a bit of a boost.
It's obvious she doesn't feel good, but there are so many things that could cause it. From her getting into something or eating something not-chicken-food (a string or piece of brightly colored plastic or discarded chewing gum or who knows!) to her catching a bug (not the tasty multi-legged kind) to there being something wrong or injured internally.
The symptoms don't indicate egg-binding, I agree. The abdomen being full, as it's not hard, could have been from her just needing to drop a bomb, so to speak. That leans towards a partial blockage, which may just take time to resolve.
I would continue to keep her separated and quiet, continue to offer food, grit, and calcium, and "pump" the vitamins/electrolytes to help boost her strength while she tries to get past this problem .