I fell, since then my leg is painful behind my knee. it radiates up and down a bit. it is worst when fully extended and when changing positions.

Knee sprain: Ice for 20 minutes every 4-6 hrs for 1st 3days. Rest the knee Ace bandage for support and swelling Elevate knee if swollen If pain,swelling,weakness,knee buckling or instability occur or persist,see your doc or an Orthopedist for further work up.

Sprain: Sprain of muscle or ligament is likely. If there is swelling, discoloration or severe tenderness, have your physician examine you. Otherwise, try cold applications for 48 hours and then heat; NSAIDS like ibuprofen and wearing a ACE knee brace should help.

Knee injury: You may have injured a bone in your knee or strain or torn a ligament which will create pain and instability in that knee with mobility. You may need to support the knee with a brace, apply ice to keep it from swelling (if injury within last 2 days) and see your doctor for a physical exam and possibly an MRI to detect torn ligaments which may require surgical repair. For now, minimize mobility.