It could be an injury, but not likely. An injured leg usually still works and the chicken will limp or hold it up under their belly because it hurts.
More likel this hen is becoming symptomatic from an avian virus, either Marek's or avian leucosis. I had three pullets two years ago with the exact symptoms. I documented it here.
https://www.backyardchickens.com/th...update-now-another-pullet-going-lame.1432738/ With each pullet it began with one leg stopping functioning and the chicken walked by holding it out in front.
What happens is the sciatic nerve becomes inflamed, and the leg stops working. Then the other leg is involved, then the rest of the body. My pullets all were super exposed to a broody hen with leucosis, becoming infected from her shed virus laden dander as they slept under her.
If this is what your hen has, it will continue to get worse despite everything you try to do for her. If you read through my thread, you will get some ideas as to everything one can try when confronted with these symptoms. I treated with everything from vitamins E, B-2, B-6, B-12, vitamin E and selenium, antibiotics, and even an anti-viral med. None of the affected pullets survived.