there are several women i know, myself included, who gained a cup size or two or three in their 30's and it wasn't connected to pregnancy, weight gain or surgery. they just grew. like they took some time off between the ages of 18 and 31 and then POOF.
You're definitely not alone, I'm also 20 and have the EXACT same problem. If you figure out the problem please let me know.
It might be also that your doctor will point to your age and say you still might have some development. If this is definitively not true of all your female relations, I would press for a more thoughtful answer.
I am in a UK size of 16, and a US size of 14 - so it isn't because i'm too thin, Haha =P
And every woman in my family has been in atleast a C cup - No one in my family has every been "small"... in breast size or body shape. This is what was concerning me.
If you're the only woman in your family with this profile, I'd check in with the doctor, but given that you got everything else puberty-wise, it doesn't really sound like your hormones are out of whack or anything. The only things that ever affected my bustline were weight gain and pregnancy. Not that I recommend the latter just for developing the bust, but if you are really slender, you might consider putting on maybe 5 or 10 lbs. to see if there is any difference in what Church Lady used to call the "chestal region." Good luck!