have you treated her for mites?
She could have a case of scaly leg mites and they are targeting her because she is weakened. It takes a while for them to deposit enough crud to lift the scales. For now they may just be biting. Apply ivermectin spot on to her neck and grease up her feet every couple of days with coconut oil or vaseline, and do this for a few weeks. Treat the whole coop with an insecticide like permethrin (household flea spray).
Your temps are not too cold for chickens. That's a warm winter! Do they have a draught free, well ventilated roosting area? Maybe post pics of the inside of the coop.