Mareks is usually progressive. In other words, if you have been watching your flock, you would have seen her get worse and worse over time. If this just turned up in the last day or so, I would suspect injury of some sort.
Give her an over all exam to make sure she doesn't have any external injuries. Do you keep a rooster? If so, look under her wings, behind the neck, abdomen too where the spurs might have punctured her skin. Feel her feet, foot joints, hocks and up to the hip. Any heat, redness or swelling? If she broke something, it would definitely be hanging. You can usually feel dislocation too. Compare her hock and hip joint with the opposing one. They should feel the same. A dislocation will be sticking out pretty badly.
If there isn't any swelling,heat, redness or inflammation, it is possible it is Mareks. Look into this birds eyes. Many time with Mareks the pupils are not round, but oval or out of round. Compare eyes with other birds eyes.
But for now, I would confine her for possible contagions, and also if this is a simple sprain or strain that she might heal by not using her legs as much. Keep her in an area where food and water are close at hand so she barely has to move to get them. Sprains can take 4 to 6 weeks to heal. If this is Mareks, she will go down hill pretty fast in this amount of time, slowly becoming paralyzed all throughout her body, eventually no doubt leading to death.
Good luck and keep us posted!