Over at GoodTherapy.org there is a nice little post about the difference between being socially anxious or simply introverted. Social anxiety can be a disorder and typically causes the individual distress; on the other hand, introversion is a personality trait or style that causes the owner little, if any, distress.
The article makes clear that both introverts and extroverts can have social anxiety. Introversion/extroversion tends to have a genetic component. If you come from a gregarious family, you're likely not the only one. Similarly, if your family is more retiring, you are more likely to be so, as well.
Sometimes introverts might be told there is something wrong with them, especially if they come from a more extroverted culture and family.
In the article linked above, you'll find some simple questions to help you know what is anxiety and what is personality. I think it's helpful to know which is which.