If I'm the only one in my extended "family" with (just a trace of) social anxiety, does that mean I probably was adopted?

No. I think it's caused by your upbringing. If you were neglected, or abused verbally or physically. Scared into getting a bunch of unreasonable fears, like fear of dark and whatnot. All of that piles up, and by the time you get in school, if you start getting bullied in there also, you get even more of that social awkwardness.
Also another reason could be if you have a super arrogant best friend, that huggles all the attention and pushes behind, that could keep you from developing good ..or at least average social skills.