You may have injured your knee during a football match. Achilles tendinitis or patellar tendinitis are the two most frequent causes of pain in the knee after an injury. Further, you may also have swelling or a problem moving the knee. To alleviate your knee problem, rest your knee, then put some ice on it to let it heal and elevate your leg. If you are still experiencing pain, it is relevant to take a rest from your daily activities and consult an orthopedist or therapist.