I agree with your assessment that it may be a pulled or sprained tendon or ligament, most likely jumping down from a high roost or other high place. You're right, you shouldve isolated her when the incident first occured. Walking and running around since the initial injury has made it worse.
Do NOT give her any types of antibiotics, by your description there's nothing to indicate a bacterial infection.
The best thing you can do is keep her isolated in a cage or crate for rest and relaxation, limiting movement. You dont want her up walking around causing more serious injury to her leg. You can continue adding vitamin B complex to her feed which might help speed up recovery. In a weeks time release her from the cage and see if there's improvement, if not, put her back in the cage and continue the vitamin B complex in her feed. Take her out of the cage again in one week and see if there's improvement. If not, recage her and stop the vitamin B complex in her feed. I've dealt with plenty of these types of injuries. It takes time for leg injuries to heal. They can take a week, a month, sometimes several months to heal, or never heal at all. If she doesnt heal, it then becomes a quality of life issue, then you have to make a decision whether to continue caging or culling her. Releasing her with the other birds will cause fighting, aggravating the injury and upsetting the pecking order. I've had a very good success rate treating hens with leg injuries such as your hen, not so good with roosters.
However in your case if your hen had been caged sooner, she would have a better chance healing and recovery.
I recommend that you lower roosts inside your coop if they're too high and eliminate other high places in your yard that they can jump down from to cause injury.
