Poor thing matured physically faster than she did mentally and I'm really wondering (like Frankbtl) if ... something happened that she's not talking about. It may be nothing, but women who mature quickly often get some negative attention early.

My wife was in a C-cup by age 12? 13? Her grandpa always used to call her Busty and stuff at that young age. She's still very cognizant and embarrassed by her large breasts because of the inappropriate attention she got from her grandfather.

It may be as simple as that.