I get a sharp pain in the right knee when I move the leg quickly or when I get up from a chair. I often cannot move or bend my knee, but after few steps, the stiffness and pain eases. What could be causing this?
There are quite a few things that can cause this. A torn or inflamed ligament can cause your knee to lock up. You could also have problems with the bursa or fluid filled disc that provides lubrication to the knee. You might want to try Health Centrals 'Symptom Checker" to see if you can narrow this down more but you'll probably have to see a doctor to figure this out.
You might try rest, anti-inflammatories, and ice to that area to see if it improves. Another cause of stiffness like this arthritis, which can cause stiffness in the knee if it's held in one position for a long time. Here are some links that might be useful, in any case:
7 Surprising Facts About Arthritis
Everyday Ways to Protect Your Joints
Why Yoga Can Help Soothe Many Types of Chronic Pain