“What causes joint pain in knees?”

Multiple causes can precipitate knee pain: joint/bone issues, soft tissue disorders, bursitis, injuries, ligament tears, etc. Self-diagnosis, via google search, will help you. Seeing a right doctor will. The best thing to do is go see a doctor, preferably a Physical Medicine & Rehabilitation specialist (a doctor who specializes in bones, muscles, nerves, brain, spine issues).

Knee pain is multifactorial. See a sports med Dr.