What causes you to shiver unexplainably?

Question

So you're just sitting there, having lunc with friends, and suddenly you'll raise your shoulders, lower your head and a shiver will race down your spine. The temperature is still quite warm, and nothing environmental seems to have triggered it, and yet you get "bad vibes". I've also heard the term "Someone walked over your grave".

What causes this interesting reaction? Is it some evolutionary change that is now redundant?

Answer 1

I don't know. I've shivered unexplainably before, but I don't know why... how could someone "walk over your grave" if your not dead?

Answer 2

Maybe low blood pressure?