What are possible causes of knee pain ?

Many, many: Injury, overuse, meniscus tear, arthritis, ligament injury, tight muscles and capsule, referred pain, loose bodies, sprains/strains, fracture, gout, connective tissue diseases, etc.

Muscles, not joint: Janet g. Travell, md the matriarch of pain therapy noted in the 50-90's something interesting about joint pain. She stated that any joint area stiffness, aching and tension was due to muscles not the joint proper. So her myofascial therapy protocols are designed to treat all types of "joint pain." begin a self-care self/pro massage, chiropractor, heat, epsom soaking, stretching. I use acupuncture!