What's wrong with my chest muscles?

First off I am a skinny guy. Im right handed too. My right chest muscle is full and round. My left chest muscle isn't. It starts where it should at the top then goes down my sternum about 2 inches then kind of a curved line to right above my nipple. Everything below and to the right of my nipple is nothing but bone. Ive been working out alot here lately to try to change this. Its nearly as strong as my right chest when doin chest workouts. I don't think I was born without a left chest muscle, but more I think about it the more i don't really remember having or not having it. So my question is "Whats wrong with my chest?"