Hopefully the vet can see her.
Has she been drinking? I would offer her a little warmed sugar water or electrolytes. Get her hydrated first, then see if she can manage a little wet feed.
The trembling is concerning.
They do have bones around the vent, while the article is about checking to...
Sometimes when a hen is coming out of molt and going back into lay they may have some difficulties.
It could just be a glitch that will work itself out. It's a guess that she may have a soft shelled egg that she's trying to expel.
She's pooping, so if there is an egg, right this minute it's...
Do you have any Caltrate or TUMS? You can give her either of those.
With her being able to poop, an egg is not blocking the cloaca, but she may be having trouble with a soft shelled egg.