I also get them at night when I wake up even if I`m nice and warm, and when I mentioned them to my GP I was told that if I get them again to take my temperature and do a specimen in a bottle for him to see as I may have a urine infection, but I haven`t had any since......strange that!!!! Hope it`s ok with you now.
I think Pasta's on the right track. Shivers when you haven't got a cold, can be due to stress and panic attacks. Also related to conditions brought on by too much alcohol.