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She sounds like she could be egg bound. Here's a great link on treating egg bound hens.
https://www.backyardchickens.com/a/egg-binding-symptoms-treatment-and-prevention
Egg binding is when an egg gets stuck on its way out the vent which clogs up the hen. Usually it’s a larger than normal egg that gets stuck. Other causes include: Too much protein in her diet, too little calcium, worms, lack of food or water or low quality feed.
Signs of a hen who is egg bound include her being fluffed up with eyes closed, tail and wings dropped and lifelessness. Egg binding needs to be treated immediately and carefully. First, before bringing the hen to it, get a warm bath ready. Then carefully and calmly soak her behind and back area for several minutes. While she is soaking, gently massage her back and vent area to help encourage the egg out. Then take her out and keep massaging the area. Apply a little bit of Vaseline or vegetable oil to her vent and keep massaging. Finally, dry her feathers off and take her back to the coop where you may want to go ahead and place her calmly in a nest box with several soothing herbs to help relax her. Make sure she is warm, dry and has plenty of water. If the egg has still not come out within 24 hours, repeat the whole process over again.
Try and make this process as stress free and soothing as possible. It’s best if you have a partner to help you with this process. If your hen panics and you grab her tightly you could break the egg inside of her, which will probably kill her.
She could also be just cold. Make sure your coop is warm and has dry bedding, ventilation and no drafts.
Bedding that is warm and absorbent is also necessary. Wet bedding mixed with the cold temps and wind chills equals bad frostbite. So either use straw, shavings or shredded paper. Straw is possibly the best bedding to use in winter. Line the nests with straw to help prevent eggs from freezing. Bales of straw help act as insulation and keep the coop even warmer.
Best of luck to you!