Look up BURSITIS on wikipedia. The pockets of fluid that protect the knee from continuous friction become overstimulated and the fluid builds up, leading to inflammation in that area. There are several areas about the knee that contain these pockets. The pain comes from the enlargened pocket. Treatment is to apply ice for 20 minutes each day and make sure to elevate the knee, and to give your knees a rest and do not exercise in order to give the knee time to heal, additionally, to perform exercises to strengthen the thigh muscles and to take anti-infalammatory meds such as NSAIDS. Prolonged stress to the already inflamed knee will cause scar tissue to build up overtime if measures are not taken to take care of the knee. Scar tissue accumulated will increase your bouts of bursitis and make it constant. An additional cause of bursitis is obesity and/or bow-leggedness, flat-footedness, and other such structural abnormalities of the feet.