swollen hock, help!

    Hi, 
      I have a 15 to 18 week old Orpington. I got her about 4 or so weeks ago. She is alovely healthy hen or what I thought was healthy I kept her in avery large area with about 20 other hens, all between the ages of 15 weeks to 2 years of mixed breeds. 
In this area there was much to do for them like scratching under the shurbs and trees, going into the bushes. The shed is raised well of the ground so the hens have to jump up the steps (just three steps) to go to bed. They all have been getting on very well together and have had know problems untill now. This little Orpington called Orby. She had just started limping.
so I had a closer look at her and found she had a swollen hock joint. There was know sign of heat in the joint just alittle swollen.
I popped her into the nursery where there was nothing she could jump up on to and down again so not to put any pressure on her joint and it seemed that she was getting alittle better for a few days. This morning she seems to be limping more then ever on it, still no heat just a little warm but apart from her joint she seems fine.

I've looked up swollen joints and all I can find is Mycoplasma, which is a very typical sign of Mycoplasma but she doesn't show any other signs. 
Does anyone know what it could be or do you think she just needs plenty of rest. I keep telling her to go and put her feet up but she doesn't listen