Since my 2 year old niece felt while running her left knee is getting swollen. did all kind of analysis doctor can't find a answer. what can it be?

What kind of testing: was done? It sounds like, depending on when this happened, that in addition to being examined, she might need an X-ray, and possibly an MRI and/or blood work to figure this out. If her primary care doctor can't find an answer, she may need to see a specialist.

Effusion: Could be a reaction to trauma. Give her ibuprofen and keep leg elevated when lying down--if not better in a week or so see the dr again