Pain in my knee cap i've had it for months it hurts when i put my leg straight and when i go down and up stairs, what could it be?

PFS: Patellofemoral syndrome, usually more common in young females. Sometimes also common in runners. Treatment focused on quad strengthening exercises which leads to your knee cap tracking better as it should on the groove in your knee as you bend and straighten your knee. See your doctor, you will likely benefit from some physical therapy, and nsaids.

Patellofemoral pain: Usually this is pain from abnormal tracking of the patella (kneecap) in the groove on the femur (thigh bone) at the knee. Bracing with therapy is my usual treatment. Occasionally there is a severe maltracking of the kneecap and realignment surgery is useful.