Your older chickens are likely immune by now. These new ones you brought in are likely becoming exposed now. It doesn't matter that the others are fine. Coccidiosis does not hit all chickens at the same time. She is displaying signs, the ruffled feather look, not moving much, not interested in eating. It won't hurt your other chickens to have it, so I would put it in the water and treat for at least five days. Change the water daily with new medication. Afterward, put some vit/mineral supplement powder in their water for a few days. I would give them the vit/min supplement at least a few days a week in the future. It mixes in their water and you can usually find this type of powder in the chicken section at places like Tractor Supply.
I'm not saying it is what is wrong with her, but it is likely the first thing I would try given that they are new arrivals to your flock. If she was wormy, she would feel boney and I would think still may have an appetite, and if it was a bad worm load you sometimes can see them in the poop. It could very well be something else wrong with her, a viral or bacterial infection, but those are very hard to diagnose. I'd try the corid first and see if there are improvements after three days. Make sure she is drinking even if you have to dip her beak in the water to get her to drink.
