quick question

[lifelessnbroken.livejournal.com]

okie this is probably stupid but why is it when you tell someone your name and you say 'а тебя?' but when someone asks how you are and you say fine, and then to ask them how they are u say 'а у тебя?' is it just a formal difference?

or maybe i just have this all wrong. Я очень устала. (is that how you say im very tired?)

thanks