Pain and swelling of the knee which has lasted for three weeks, needs to
be investigated. This is due to
inflammatory reaction to injury
or disease. Collection of synovial fluid in the joint causes the swelling. However the risk of problems with the cartilage or meniscus should be ruled out. Consult an orthopaedic surgeon for an evaluation. Rest, immobilization, anti-inflammatory drugs, injections of steroid into the joint will help.
Take care and good luck.