Sometimes out of nowhere my body will get really cold (shivering) for 5-8min then either really hot or goes back to normal. Why does this happen?

Not enough info. In order to give yoiu an answer i would need a lot more information. That would entail seeing your family physician and perhaps have some blood tests done. Are you taking any medication or herbal preparations? Lots of things can cause this mostly benign but to be sure please see your physician.