I have a very sharp pain in my knee! help?

[jed]

Recently at football, i was running and my knees started to hurt very bad, as i ran a bit more, it felt as if my knees would no longer support me. i sat down to strech and nothing happend no difference, i got up and could barly walk. The pain i felt was from the upper part of my shin, to right at the bottom of my knee cap to the top. Its a very sharp pain that hurts a tone! After practice i sat on a bench and a bit of pressure was appplied to the back of my thy( of what ever is on the back side of the upper part of my leg) and the sharp pain shot back stronger then ever. Now every time i apply pressure to the back of my thy it kills and i dont know what to do! what is this that i am experiencing, and how do i treat it? thanks