Random Excruciating Pains In My Legs?

so every few hours i get this random pain in the middle of the bone under my knee. the name escapes me. if you have ever seen twilight when bella gets her leg broken in the studio thats the only thing i can say to discribe it! it brings a tear to my eye and it is the worst pain i have ever felt! it has been there for about 3 years always presuming it would disapear as i grew. i am 13 years old and i cant stand it! it hurst so bad. it literally feels like a blunt object going through my leg! i dont know what all to do. any explenation or theory would be nice thank you