Basketball puts a lot of stress on the quadriceps and the patellar tendon from the running and jumping. Change in direction and the constant pounding while running can damage the cartilage, the ligaments and the meniscus. Any of these structures could be injured, without having a memorable trauma this probably fits into the straw that broke the camels back category. Following a physical exam, your doctor will probably recommend diagnostic imaging. My guess is a muscle imbalance in the quad.

William Krieger