ok. my strong suggestion would be to see a doctor. possibly get an MRI or CT of your knee. it is very possible that you might have torn your ACL, or Anterior Colateral Ligament, not a lot of swelling, and yes, it's very possible for you to still have functionability with it torn. you are experiencing the same pain when you provoke it in a very physical manner. which suggests the tear. this is one of the most common tears of the knee without disabling you right away. surgery is the fix. im not telling you that you've torn it, but from what you posted thats what it souds like. your doctor can tell you more, and i would suggest seeing him. if it's torn all the way it can't get any worse for that ligament, but the longer you wait to check, the more scar tissue would build up, and if it is the acl, then that means the more they have to take out. get it looked at by a professional.