this sounds like an i.t. band problem. it is the most common athletic knee injury, the major symptom is pain in the back of or side of the knee. the tendon runs from your pelvis down to your knee. the problem is partly caused by running or climbing/hiking until you are tired, which causes your knee to bend slightly inward. i had this problem, stopped running for two weeks and got new running shoes with inserts, haven't had it since. it happened when i increased my milage too quickly. there is also an elastic band that goes around your leg that is supposed to help this problem. there is info on the web about this. take ibuprofin and cut back for two weeks. ice helps too.