Anyone know what I could've done to the back of my knee?

I play left wing at soccer, and lately every time I go to cross the ball in I have a sharp pain on the backside of my knee and it's progressively gotten worse. Although today when I crossed it in it hurt worse than it normally does. It hurts terribly to straighten it out completely when I run, also. Is there anything seriously wrong with it do you think? Cause I don't want to go to the doctor if it's nothing serious.