What can be the cause of (constant) pain r knee that increases with bending it &alternates in left side sometimes up , down, back , around knee cap?

Meniscus tear.: It could be a meniscus tear. The meniscus is the cushion between the thigh bone and the shin bone. You should be evaluated by an orthopedic knee specialist.

Knee pain: Could be chodromalacia, inflammation of your knee joint, an overuse type injury, etc. See a dr. And get evaluated and then the treatment would be forthcoming. Good luck, get well soon.