Some people are just naturally skinny. I have a friend who is the same way. Its nothing to be worried about. Insecurities are hard to overcome but just remember every body is beautiful!
Have your endocrine system check. It can see if it's a hormone problem or Turner's syndrome.
Some people are by nature small chested. You are petite all the way around and it is unusual to be very petite and have a large bust. You get your period on a regular basis?