Well it depends. What happens when you try to speak in front of people? Cant get the words out or stammering or something else? Because if its that its probably in the brain. Stuttering is a physical thing and not because of just stress or being shy. Those factors make it worse but the origin is that some connections in the left part of the brain arent working like they should. If its more like you cant find the words or feel flustered then i think it could be anxiety. I am not a professional so i am just guessing here and coming from experiences.