HI,
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.