I feel you. I'm TINY. I actually think I'm not underweight, I think last time I got checked up on this I was found to be "normal" for my height, but still, I'm tiny. I'm about 5'1 and under 100 pounds. Most of the time I feel pretty healthy and for some reason I don't feel this way when I see myself in the mirror, but WOW, there are some times when I see pictures that kind of shock me! I saw a few that really made me think I looked way, way too thin.
I keep thinking to myself that I have a "goal weight" of being able to give blood. I'm not heavy enough to do so right now.
It doesn't bother me most of the time, but eh, I'm just small all around, with no chest to speak of. I don't feel too good about my body when it feels like I have no shape, and I'm just thin.