What could cause knee pain ?

Many things: Arthritis, meniscus tear, acl tear, ligament injury, cartilage injury, inflammatory arthritis, gout, tumor, cholesterol medicines, muscle strain, bursitis, vascular insufficiency, overuse injury, insect bite, viral effects, infection, blod clots, metastatic disease, stress fracture, sickle cell disease, a vascular necrosis, and the list goes on. Need more info for better recommendations.

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!