I had a chicken who acted much like that after an incident of eating onions. Maybe she got into something to eat that she should not have? Mine survived fine, still have her a couple years later.
Now the hen is not doing well again. Her face is pale, she's taking it easy, and she's not walking around with the other chickens. I think I'm going to de-worm them in case that's it...I'm not sure what else to do.
Yesterday she was in the coop and I thought she was laying an egg; she came out to eat some BOSS I gave them, but then went back inside. When I looked in, she wasn't on the nest, she was just sitting down. The hens never spend time inside unless they are laying, so she must not be feeling well. Today she was hanging out underneath the coop and not walking around with the others. I don't think all the hens are laying right now either, so maybe they all have worms...
Is it hot there? My hens will lay down more when it is hot AND stop laying. For them, "hot" means any severe rise in temperature. They stop laying when it hits around 85 degrees here, because it is usually much cooler.
My chickens also go into the coop to rest when it is hot.
Could she have gotten into the same something that made her sick before? Do they free range?