What Causes Pain And Stiffness Behind The Knee And Leg?

pain and stiffness behind knee and leg and also back pain around waist on left side, sometimes from pain in the same area. I have trouble sleeping, I teach two classes from 11:00 --3:00 and after that am exhausted. I am obese, have GERD, narrowing esophagus, arthritis and have always been a relatively active person; however, now I am slowing down considerably and rapidly it seems. I am 73 years old.