I have knee pain when going down stairs, sitting indian style, or straightening my leg when it has been bent for awhile. what could this be?

OA vs PFPS: Two possibilities are most likely. The first is osteoarthritis, or wear and tear ("rust in the joint"). This is usually in middle-age people but can occur earlier with excessive use. Another option is patellofemoral pain syndrome, or inflammation where the kneecap tracks on the femur (upper leg bone). In most people, these can both be helped with antinflammatory meds and quad strengthening.

Patellofemoral Pain: Pain in the front of the knee around the kneecap, that often gets worse with using stairs, prolonged sitting, or activity such as walking long distances is called patellofemoral pain syndrome. It can often be addressed with physical therapy. I would recommend seeing a physician to make sure nothing else is going on, and help guide your treatment plan.