It can be infection.
It can be autoimmune disease.
She needs to see an orthopedic specialist to see if there is fluid in the knee.
If so, it can be tapped for analysis.
Otherwise an x ray is useless, she needs an MRI of the knee.
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A blood test would not show infection in the knees.
You can't tell for sure if there is fluid, or arthritis, without imaging.
Juvenile arthritis is possible, and she needs at least an ultrasound of the knees, probably an MRI.
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