What is my problem? The left side of my mouth goes to one side while smiling and while talking in public.

What is the cause for this? It is more evident when I talk in public and meeting strangers...I am 20 year's old now. sometimes my face shiver also..I started to notice this when in was 14 years old.. but didn't take any treatment.now it's causing trouble.i can't talk to any new persons and public because of fear and face shivering.its not visible every time.