If it is not caused by an activity that she is doing it is probably an autoimmune response to something. That means, a food allergy, a vitamin deficiency, or other type of sensitivity. Start by keeping track of all her habits that may be putting pressure on her knees during the day, such as how she climbs, does she drop to her knees suddenly when she is excited about playing with a toy or pet? Give her warm baths with a small amount of Epsom Salts...not too much she is little, half of the adult recommendation at most. Then get her into a doctor to have some kind of scan-X ray, ultra sound, MRI, whatever she/he orders. This is important. They may not find anything, but it should be done regularly until she is not having the problem. My sister cried with knee pains for six months as a child. She used a heating pad for relief. When my parents finally got worried enough to take her in, she had a tumor protruding out of the side of her knee cap and needed surgery.She has had an ugly scar all her life. Not everything is someone's fault, sometimes we just need extraordinary care. Don't panic, because you will figure it out, you know her best. XO L.