Very sad or scared chicken? Help!

Ok, I know this sounds crazy, but I think I have a hen who's afraid of being outside. Let me explain. My chickens were received from mail-order in late March/early April. Being that it was still way too cold here in Wyoming to put them outside and being that the coop wasn't built anyway, be housed them in our heated shop. As they grew, we expanded their living quarters as needed, all inside our shop (which is rather large). Anyhow, my husband finally finished our chicken coop and fenced in yard. We put the 17 hens and one rooster out with our 2 ducks (about 3 weeks older than the chickens.) They share a yard but have different housing.
All went well for the first 5-6 days. Then I noticed my favorite hen "Rita" wasn't coming out of the coop. I kept an eye on her until finally i couldn't take it anymore, she was lethargic, droopy, just sad acting. Didn't notice any insects, discolored poop, limp or discolored comb/waddle or scaly feet. Of course, I separated her from the rest of the flock (right back into the shop) to prevent any spread of disease if there was any. I treated everyone with ACV, electrolytes, and meds.
When I first took Rita back into the shop, I gave her water by hand. She was obviously dehydrated and took the water happily. Within 24 hours she was acting alert and happy again, so I decided to put her back out with the flock. The next morning, I found her with her head stuck under the duck house like she was hiding!
She isn't "pecked" on, she gets along with everyone including the ducks. She just seems so scared she won't eat or drink and compromises her health. I know it seems nuts, but has anyone else out there every had this happen? And do you have any suggestions?

Thanks for your help.