I agree with the others. The panting, breathing hard sounds like she is really stressed out. Is she completely alone? No other chickens next to her? Make sure she has someone next to her to lower her stress levels. Egg bound sounds causeable. At this point it is a wait and see situation...
Poor girl legg injury can be difficult to diagnois over the net. If she is just walking with a limp thats a good sign that it is probably not broke but more of a muscle/tissue injury. Since you haven't found any injury on the bottom/top/toenails foot. Does the foot feel colder then the other...