Knee pain usually results from overuse, poor form during physical activity, not warming up or cooling down, or inadequate stretching. Simple causes of knee pain often clear up on their own with self care. Being overweight can put you at greater risk for knee problems.
Knee pain can be caused by:
Arthritis -- including rheumatoid, osteoarthritis, and gout
Baker's cyst -- a fluid-filled swelling behind the knee that may accompany inflammation from other causes, like arthritis
Bursitis -- inflammation from repeated pressure on the knee, such as kneeling for long periods of time, overuse, or injury