I would seperate her asap. I am not the most in the know on chicken illness but thats always the first step.
More information might be helpful for the others that are sure to chime in. Any discharge? When you bring her in and offer her water is she drinking at all? How is her comb. Sometimes lack of oxygen makes that blue. Is she stretching out her neck when she breathes. Like she is straining. Any resent deaths in the flock. Any changes in feed?
If it were me I would bring her inside in a box, I have probiotic powder i put in water and offer that and wait for an answer from those who know lots more.