What Causes Loose Knee Joints?

My Knee joints seem to be loose after a walk.

And my left leg knee joint pains when am sitting cross legged,

I mean when there is a weight on the leg(only sitting pose).



I am only 22. I suppose this is because of some over exercises.

In order to reduce my thighs, I worked on skats and some martial

arts.



Can you help me in finding What the problem might be ..?

And what are the tests I have to take, to check if this is a beginning

of a ligament injury.