What causes this? An odd experience

As far as I'm aware, shuddering is an emotional response. It has happened to me both positively and negatively, whether is was something that wierded me out or some kind of revelation that really helped me understand something. Often times we perceive more than we give ourselves credit for. Sometimes these perceptions are correct, sometimes not, but in either case our body gives us a heightened sense of awareness.

I have a group of friends that when we get together it happens all the time to all of us. Can't give you an explaination of it. I don't necessarily think of it as being dread.