Well, without a physical examination and probably a few tests, it would be difficult to point out a cause. Few possibilities include infections/ inflammations, arthritis, bursitis, tendonitis, cyst formation, nerve impingement/ injuries, iliotibial band syndrome, injuries to the internal ligaments/ menisci, osgood-schlatter disease, bone growths/ tumours etc; just to name a few. I would suggest consulting an orthopedician for an evaluation; which may consist of a complete physical examination and a few tests like routine blood tests, imaging studies like X-ray/ MRI etc. After a cause is diagnosed, it can be managed accordingly.
Hope this was useful.