4 week chick with swollen "knee"

[Kasai]

Hello,
I have a 4 week chick who has not been wanting to walk for about a week. Didn't like putting weight on her left leg. I noticed its gotten pretty swollen and she won't put any weight on it at all now.
She's been separated for a week with just 1 friend, none of the other chicks are having this problem.
Can anyone please advise me on what's wrong with her and how I can fix it?

 

[black_cat]

What does the swelling feel like? Does anywhere along the rest of her leg feel 'off' to you? Anything out of sorts aside from the joint?

 

[Kasai]

Her skin feels warmer than seems normal. The joint feels hard and warmer than the rest of her leg.
I was checking her over and noticed she has this second bump and some yellow speckles on her legs. She's an orpington so shouldn't have yellow on her legs.
I took some better pictures

 

[Kasai]

You can't really tell in the pictures but her entire left leg is swollen to a size noticeably larger than the right leg.
It doesn't seem to hurt her when I touch the bumps.