Sharp pain in Knee...can some please help?

Every once in a while i will get these sharp pains in my left knee...when i get them it will be all day...its when i stand up on my tip toes or go from standing in one place to walking this pain shoots through my knee and it feels like my knee wants to give out...im a bowler and my left foot is my sliding foot so this knee is often under extra pressure...this ever happen to anyone? Do you know what it could be?