In Right Leg Only, When Trying To "fold" Lower Leg

In right leg only, when trying to "fold" lower leg back to upper, a pain runs along outside of leg -- above, adjacent and below knee. So "stretching" (gentle holding of leg in folded configuration) lessons pain but has not ended it over a few weeks. I am regular exerciser with walking, biking, and elliptical machines being my favorite. What is cause of pain and what could I do to relieve?