What Causes Burning Pain Radiating From Back Of Knee To Heel?

AFTER WALKING I GO TO BEND LEG TO SIT - SHARP BURNING PAIN BEHIND KNEE - ALMOST IN TEARS, PAIN MOVES DOWN PACK OF LEG AND INTO SOLE OF FOOT. PAIN DOES NOT STOP WHETHER STANDING OR SITTING. STARTED AROUND 1 PM TODAY - IS VERY PAINFUL RIGHT NOW. I DO HAVE SOME RELIEF AT NITE WHEN SLEEPING WITH FOOT ELEVATED. TOOK SOME PAIN PILL (WILL HAVE TO LOOK UP NAME)--hasn t helped . I m almost afraid to move my leg and try to stand! about 3 weeks ago I had an ultra sound to be sure I didn t have a blood clot -I did not haveone. Don t know what to do now.