Now I am no vet, and there are good comments here on monitoring the condition of her crop. When you feel of it first thing in the morning, there should be nothing in it but perhaps a few pebbles. Roughly small marble size. If you can see the crop sticking out, or if there is more in there than that first thing when they come down from the roosts, there could be a problem.
I am chiming in because her symptoms sound just like my own EE girl a couple of years ago when she started laying. It was about the same time of year, too. She was absolutely lethargic and miserable for several days before lay. She crouched, her bottom pulsed every now and again, and she didn't want to do anything except sleep in my lap. None of my other girls did this, and she was the last of the bunch to start laying.
When she finally laid her egg after an agonizing 3 hours of chickeny labor, it was the biggest egg I have yet seen from my hens.
Anyhoo, if you can't find anything else wrong with her you might look to that. She's the right age. Keep her warm and comfy as possible, make sure she stays hydrated (half-strength Gatorade is what my vet recommends for bird stress), and keep an eye on her.
If more experienced folks have other info, please chime in.
OP, keep us posted! Good luck.
Ed. to add - - kudos to you for keeping such a close eye on your hens! Good job.