What Causes Acute Knee Pain With Inflammation And Infection?

My 9 year old daughter has had knee pain for 4 weeks not. Her CRP and ESR were high: CRP=1.5, ESR=35. Indicating inflamation or infection according to her Dr. This knee pain began suddenly about 6 months ago. Was gone after a about 2 months and recently returned and has been present for 3 weeks now. She wakes up in pain and stays in pain all day and worse at bedtime. Her Dr. is baffled. X-rays of her knee and hips have been done with no indication of the cause. She is scheduled to have another bone scan done, which was already done 5 months ago and it was clear. She has a very high threshold for pain, due to many past bone fractures from sports injuries so I don t feel she is exagerating or making it up. She is depressed from being in pain 24/7, I give her 2 xtra strength tylenol at a time. She is tall for her age and also slightly over weight all though she has not gained any weight in a year according to her Dr. Do you have any suggestions to what might be causing this. This happened all of a sudden. No injury either time, she woke up with the pain.