There are a whole lot of possibilities. Is she perhaps too hot? Chickens can die at 100 F from heat exhaustion. They need shade, breeze and plenty of cool water. If they are overheated, they will pant with their mouth open and hold their wings a little away from their bodies, before they get to the lying down stage. In a pinch, you can dunk the bird in a bucket of water and let her air dry. There are a number of things people use to cool their birds -- bottles of frozen water they can lie next to, ice in their drinking water, misters, wetting the soil they walk on down. a wading pool they can stand in (if they will,) etc.
Especially if you live in a warm, moist climate like I do, you might consider worming her, if you haven't. I prefer Valbazen becausse it gets all worms and is relatively easy on their system. You can get it online at Jeffers Livestock or
Amazon, if it's not in your feed store. It's a cattle or other livestock wormer. You give 0.5 ml by mouth, repeat in 10 days, toss eggs for 21 days. You can squirt it in their mouth, or put it on a bit of bread. I would worm the flock.
If you haven't, check for lice /mites and treat if needed. A bad infestation can get them quite sick, even kill them. They come from wild birds.
I always think of heat plus these two things because they are treatable. It is quite possible she has something that isn't treatable, like internal laying or a tumor. I hope it turns out better than that for you.
http://chat.allotment.org/index.php?topic=17568.0
http://ohioline.osu.edu/vme-fact/0018.html
https://www.backyardchickens.com/t/...ll-seeing-live-worms-in-poo/0_20#post_9315842
http://healthybirds.umd.edu/Disease/Deworming Birds.pdf