Hey Katie!
Welcome to our board.
I think you're feeling what lots of teens feel, just at an older age. Waking up almost overnight in what seems like someone else's body is the beginning of the journey into eating disorders for many of us. I know it was for me, although I was quite a bit younger and was teased *because* I had a figure when no one else my age did.
You sound like you're at a great, healthy weight right now. As you get older, your metabolism is going to change - that's just a given. It's normal to not be able to eat like you did when you were younger. Also, whether you plan to have children or not, nature assumes that you will, hence the extra body fat that women carry.
Instead of obsessing, just concentrate on being healthy for now. Up your exercise a little, but don't go overboard. (There's a thing called "exercise bulimia"...you don't want to fall into THAT!) Most of all, PLEASE stop making yourself throw up. It will bring you nothing but heartache and self-hatred and - in extreme cases - hospitalization and death.
You say you're 18. Are you still in high school or are you in college? If you're still in high school, is there anyone you can talk to...your mom, a teacher, a school counselor, your best friend's mom? If you're in college, visit the student health center and ask to see a counselor. In recent years, colleges have really begun to take eating disorders seriously.
Keep coming back. We want to know how you're doing.