3 year old mysterious swelling

My sweet 3 year old niece is in the hospital tonight. She was previously diagnosed with juvenile arthritis as she was having periodic swelling on one knee. She's been on a medication that treated it well but in the hopes of easing her off the medicine she went from twice a day to once and this past weekend they took her off the medicine and watched her closely. Wednesday night she got up and was in a lot of pain. She couldn't put weight on it and walk to my sister's room. Heartbreaking. Her upper thigh is swollen now. Today they ran a series of tests, blood work, xray, ultrasound and nothing turned up - the drs. said nothing is explaining this swelling. They said the muscle looks good and the joint is not swollen so they now don't think it's juvenile arthritis. Tonight she had an MRI and we will hopefully learn more Friday. We are all so confused because they've tested for everything that would be 'common' and the dr. says whatever is happening is not common. If anyone has ANY thoughts on what it may be or the questions my sister and her family should be asking please share them. We are all so concerned. Thank you.

[txsilver]

I am sorry your niece is going through this.  It is a heart break when children suffer.  I hope that someone with more specific experience to your situation will write.  I can suggest that if there is not already a rheumatolgist involved, one be engaged.  
If you know what they have tested for or any of the lab results, that may be helpful.  I hope they identify the cause soon.  Please write back.