The liklihood is you have a torn miniscus. Definitive diagnosis is by MRI of the knee. Uusually fluid is drawn from the cavity with a big needle. Don't look! It isn't very painful. Then the fluid is examined under a microscope. Do not delay in having this looked at.
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The location of the knee pain can be a useful guide. Pain in the front portion of the knee can be caused by bursitis (inflammation of the lining of the joint, possible due to injury while playing), arthritis, or injury to the patella cartilage (front portion of the knee). Pain on either sides of the knee is usually related to meniscal tears, injuries to the collateral ligaments, and even to arthritis. Pain in the back of the knee can be caused by arthritis or Bakers cysts (cystic accumulation of synovial fluid, also causes a swelling). So as you can see, arthritis can cause an overall knee pain. Ligament injury causes pain to worsen on walking, bending and stretching the knee. Meniscii too if torn or injured, result in joint instability—that is you are not confident the knee joint will take your weight. Often the person feels a popping sensation on moving the joint. It can also be tendinitis due to inflammation of tendon at the knee joint.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP and orthopedics doctor to run tests and examination to clinch a diagnosis.