Is it bruised and swollen compared to the other one? It sort of looks that way in the picture. It maybe got it's leg caught on something or for some reason got it twisted. If that's the case, I'd keep her separated and in an area she doesn't have to walk far for food/water so it rests it. You could try soaking it in warm Epsom salt water, then can take a better look at it too. That might bring the swelling down as well and make it feel better.
You could also give it/them vitamin water, like Nutra Drench or Poultry Cell. That'll help it with healing.