text: This could be a number of things, depending on your age, weight, and activity level.  Somebody who is older may experience a condition where the cartilage that surrounds bones that make up the joint begins to wear down over time and bone begins to rub on bone.  Another possibility is a meniscal injury that can result from a twisting motion while weight bearing.  Often times people complain of locking, clicking, or pain with going up or down stairs.  Patellar tendinitis can affect any age, but is commonly found in active individuals that are involved with jumping sports, but can also be found with any activity where there is a lot of motion at the knee joint and flexibility is an issue.  Osgood Schlatters affects youth and is a condition where the patellar tendon begins to pull the immature bone away from the tibia where it is attached.  Gosh, this list could go on and on, from knee cap abnormalities to ligament damage.  The best thing is to go and see your doctor.  GOod luck.