What's wrong with my knee?

I just started playing soccer keeper, but I don't know if that has anything to do with my knee(s). if i bend my left knee as if to sit criss cross or if i bend it any more than it needs to walk, then it hurts really badly and its so painful to straighten it again. sometimes it hurts to run, but not walk. it feels like it's in my kneecap almost. there is no bruising or swelling. anyone have any ideas? its been going on for a few weeks now