Is she the lowest in the order of birds? Sometimes the higher ranking birds can bully the lower ones away from the food and water. Especially during the winter when they are all cooped up together and bored. There is nothing to do and they get cranky. If a bird goes without enough access to food and water, even for a couple of days in brutally cold weather, they are going to get cold and sick.
Make sure you have enough room for all your birds. 5 square feet per bird in the coop, 10 square feet per bird in the run. I keep several feeding and watering stations in my coop and run. The higher ranking birds are always making the lower birds wait or even run them off. They get protective of food and water during times of stress. And since the higher ranking birds can't guard all the feeders and waterers at the same time, there is always places for the lower birds to get sustenance. A few days of going hungry and thirsty in cold weather can kill them.
For now, keep this one warm and get as much food and water in her as possible. If she doesn't get back to her normal self in the next 24 hours, she could be suffering from an over load of worms or even have coccidisos. Have you wormed your birds this year? Worms are incredibly devastating on chickens and do kill them over time. I would get them all wormed soon if you haven't done so in a while. Coccidiosis will have them off their feed and water, but generally they have diarrhea and sometimes with blood. You said her poop looked ok, so probably not cocci.
Do you keep a rooster? If so, give her a good once over looking for injuries from the spurs. The roos can sometimes puncture the skin under the wings or on the abdomen. This would cause her internal infection.
And while you are checking her, give her abdomen a feel. Make sure she doesnt' feel like a water balloon. (from between the legs to the vent is the abdomen. Feel on the outside and you can compare her to others in your flock)
Keep us posted with anything other symptoms!