Not an expert by any means and I'm really just guessing but the two thing that come to mind is egg bound or liver problem
Does her poop look normal? Is she laying?
Hey Jo, how old is your hen?
This next question is not meant to upset you, but. . .do you know for sure it's a hen? Roos will often blanch, then go red/purple when the hormones kick in. . .and they want to do business.
Just trying to cover as much possible.
Well ventilated? Well hydrated? Have you been able to look at the poo? Check for worms? Is the poo green? Can you look around the vent to see if their is any yellow/yolk like drainage?
Can you keep the coop door open or move her (might want to isolate her just the same) so she can have free/moving air? Could be a respiratory issue. Can you here any wheezing?
Has she been foraging, free range and/or have you seen her eat wiggle/red worms/earth worms?
Don't want to scare you, but might be a good time to read up on Gape Worm. They get it from the earthworms. Has she been or do you worm them, if so how often? Might be a bit early for that question, but would help to know.
Have you posted on the Emergency forum? Lots of good folks around their too, who seem always available. These are some of the questions that you may want to have a reply ready, should I miss your reply.
I know it's a lot of questions, but the additional info is most helpful.
Let us know.