My left knee frequently locks. when it happens, i pop it back in place. hours later, it is sore and painful to bend/extend. what can it be?

Is it both knees or: just one? How often has it been going on? Is it only while doing certain positions, or certain movements, like going up stairs, going down stairs, when sitting, when standing. It can be like popping a joint, and you are hearing it 'pop' in place. So it could feel uncomfortable when you extend or flex it. If it gets inflamed, you need to see a doctor and get it imaged.