Id give her warm water bath time so she can keep her leg moving and add the Brewers yeast to her food for the time being. 1tab per each cup of feed. When you look at her leg again look behind the knee does it look swollen more so than anywhere else? Feel gently along the back of her leg does the tendon feel like it’s in the right place feel the other leg for contrast. Also you could do some massage bending the leg into normal position. First can you bend the leg into normal position? I can’t tell you for sure that this is a niacin deficiency but giving the BComplex can sure help with muscle strength. This could be an injury or a tendon out of place so examine the leg let her swim an use the leg as much as she can so the muscles stay loose.
They have that same b complex at 
I also just put 1ml in their water. So, that’s how she is getting the B complex.