Why would one have a sudden drop in emotion and inability to smile in social situations?

Desensitization. Start regularly socializing with small, and i mean small groups of people you know personally in a variety of social situations, and slowly increase the number of friends. Then, slowly have your friends invite people you may not know personally, just one or two to start with, and increase the numbers of known and unknown people over time. Should the anxiety spike, stick with smaller mixed groups o.
Why? I think you would be the best person to answer this question, because you're the one living your life. It may help to explore this with a counselor, therapist, or psychiatrist though, who is not involved in your personal life. Such professionals could listen impartially, without needing to get you to "change" what you are feeling. This could provide a space in which to discover yourself.