Hi, and welcome.

My own thoughts are that some persons come programmed for mental illness given chemical imbalances, genetics, etc. As explained to me some with such propensity may go through life never being triggered while others may have multiple. This may describe the sudden onset of social anxiety you are now experiencing. Can you think of a possible trigger(s)?

Consider what coping skills you might arm yourself with. I like to keep a small stone in my pocket. I know longer need to touch it. It is enough to know it's there. I even brushed fingers across a seam on my pants for soothing myself in such situations. Self worth issues also affect anxiety. It may be helpful to do some work on this. Therapy for this is great as are some very good workbooks.