Are you completely flat chested, like you haven't developed at all, like Keira Knightly does have brests, they're practically non-existant, but they are noticable. If that's the case, then take advantage of your slim figure... I'm just assuming you're thin now because you're flat chested... normally breast size is related to weight isn't it? Just be confident, you can find some clothes that look great. I think smaller brests often looks nicer tbh. I'm kinda in the dance world, so I know loads of people who are flat, they all look great. If you do have a problem with it though, getting you down or something, which it seems you might, then go to the doctor - it can never hurt.