Well, sweetheart, everyone is different and their bodies have different timelines. For example, I was never more than about an A and a half (at most) until I was in my 40s, and my sister always wore a bra called "Almost A" even though her band size was something like a 36 or 38. But if you are concerned, you could ask your doctor.
A lot of what happens with our breast size is genetic. Just our natural build. It changes over the years with things like having kids or getting old like me. I have changed size and shape a good deal over the years. Are you really thin? That usually goes together. Extra fat often will increase breast size a bit. Also, you can try doing chest exercises like bench presses to build up the pecs. It helps a little as well. And bras these days have padding and shape you so try not to worry. Feeling beautiful as we are is important.
The other two ladies said it well. Everyone is different and your size could be due to genetics or many other factors. I bet you are beautiful though, and have no reason to feel insecure. Now, a days everyone has this perceived image of what beauty is and what females should look like, but that is false. The truth is we are all unique and beautiful in our own ways. Don't let the standards of what you think you should look like get to you.