First timer question…I walked inside last night from a New Zealand winter evening and as I was warming up I got the “big shiver”. I wasn’t cold but shivered anyway. Any explanations?
Welcome Jo.
You must have been cold if you were warming up. What you really mean is that you weren’t noticably chilled, which is a different thing.
What happens is that when you walk into cold air the body notices the temperature change. It responds by cutting off most of the blood flow to the skin. That way you can’t lose heat form the blood into the air and you stay warmer. That also causes an increase in blood pressure.
When you go back into the warm and the skin begins to heat up the body shunts blood back to the skin. Because the skin is still cooler than your body core (around the heart and internal organs) the blood loses heat. As this blood returns to the heart and is moved to the brain it causes the brain temperature to drop. That drop in temperature triggers the shiver reflex.
Basically it’s just that your body doesn’t bother trying to keep the skin warm outside on a cold night. It’s not going to work anyway and just wastes energy. Instead it waits until the skin can be kept warm and then it allows it to warm up again, which triggers the shiver.
Thanks 
Wow, and here I thought I was the only person this happens to! This reason makes perfect sense (why didn’t I think of it before?).
And plus, it makes me now think I’m normal & that everyone who’s seen me shiver after leaving a cold environment and entering a warm one and who’s simply stared at me and shaken their heads are the the weirdos. Yay!