What is wrong if I haven't had my period in 3 months?

There is no way that I am pregnant & I have not taken any birth control at all. I used to play soccer & dance but recently tore my ACL, so now I have to rest until my surgery. I didn't start my period until i was 15 & I'm 17 right now. When I stopped playing sports, did it effect my period or is this a normal thing?