I went through the bother of registering an account and blahbitty blah blah to say that I am the same way. A direct quote from a biography I wrote for a dating site:
"Upon first meeting me, people typically say I'm pretty quiet, closed off and generally unreadable, but that's only because I'm observing and getting a feel for you. Once I decide that I like you, you're going to be shocked all to hell with the personality flip-flop because I am loud, I am crude, and as much as you might wish I would, I absolutely will not shut up."
And also!
"People always think that I'm shy, quiet, reserved, even stuck up when they first meet me. If appear to be acting this way, more often than not, it's only because a) I'm in a setting or situation that is making me uncomfortable or b) I'm unsure about you and I'm hanging back to learn more about you before I decide if I like you or not."
What I forgot to include (and will probably edit to add) is that if I appear to be quiet, it's because I have my Tact Filter on. (Come to think of it, this actually falls under "uncomfortable situation.") I will blurt out the most amazingly inappropriate things (that I think are hilarious, but what can you do) and have learned that if I'm not sure whether what I'm about to say is going to go over well with present company, and more importantly, I actually care about whether or not I offend said company, it's easier just to just keep my trap shut. I'm being quiet because I'm deciding whether the person or people I'm talking to can handle me or not. I'm trying to be considerate, see?