What Causes Knee Pain And Soreness In The Muscles?

So i do bunch of martial arts that include kicking and ju-jitsu. All of a sudden i started pain in my knees after rolling around on the ground. instructor said its muscle sore from not use to of being on ground so much. well then i play cricket (sport) and i am a pitcher so pain kind of increased a bit. went to a doctor he said its fine and my fighting Kru' said my joints are too mushed together from stomping or something. now its to the point that if i stand all of a sudden my knees hurt really bad, unless i warm up a little. any ideas?