Not only was I outgoing when I was younger, but I still am under some circumstances. And I can switch from outgoing and apparently confident to "shut down" on a matter of seconds if something sets me off.
I'm not a quiet person, never have been. I'm not shy. My anxiety around people does not extend to performance - ask me to give a speech to 100 people and I'm up there. Ask me on a date, and you will find yourself dealing with a zombie (I shut down outwardly because of the fear), and I may never speak to you or go near you again (ok, I'm not as bad as I was now, but that is what would have happenned before I started treatment).
It's weird, and probably the main reason it took me so long to find out what my problem was and to get help. From what I know, shyness nd social anxiety are commonly linked, but they aren't the same and don't always occur together - you can have a problem with social anxiety but not be shy (like me) - and apparently be shy without having a problem with social anxiety (I talked to an old school friend about it - she was very shy at school, but never experienced the anxiety problems I have, and grew out of her shyness in her late teens).