Could just be strained. I had a pullet do the same thing last year. She started limping out of the blue and I couldn't find anything wrong. I isolated her in a dog crate in the coop so she wouldn't move around as much. After a week or so of R&R, she was right as rain
You have roos right? They will keep mounting and injure her more. I have an Exchequer Leghorn hen doing the same thing. I pulled her out and she is trying to use it today but I think it will be awhile before she can go back in with a rooster. Roosters can injure them and then it keeps getting worse.