I'm very much like you. My primary school teacher used to describe me as two eyes peering out from under a rock.
Haha, reading that made me imagine you as a shy frog just sitting there secretly staring.
Do you suppose people can be naturally shy or, do you think it's trauma or bad experiences of some kind that causes it? I guess it can be a mixture. Maybe not terribly confident genes paired with bad life experiences that frighten us further.
I believe some can be naturally more inhibited or introcverted, yes, where they refill their energy by having alone time. But that is very different from social anxiety.
I was mad when I just learned literally only a year ago that there is a massive difference between being an introvert and/or shy and having severe social anxiety. Because I was always told as a teenager and during college and at job training that I am quiet and introverted in a negative way, so I took that to mean it's a flaw in my very personality and it's unchangable. But instead this whole time I was actually sick! Mentally ill!
Social media and culture also abuse the term "introvert" making it overlap with social anxiety. This causes people suffering from social anxiety to not get help, and get told to just "put themsleves more out there" and "get out of the comfort zone" as if it's their fault they literally feel ill when they have to do normal things like making a phone call or saying good morning to coworkers.
As to why someone develops social anxiety, I don't know. In some cases trauma maybe, but in my case I was not abused or traumatized or neglected as a child and teenager. I do however know why I became quiet, it was because I was a year older than others in my class when I was 12-13, so I started puberty as the first classmate. It made me wanna hide more, so it stuck with me to wanna hide. I wasn't bullied but in my mind I just felt different.