This isn't my expertise, but my mom's a nurse. Your body temperature fluctuates several degrees during the day depending on what you are doing. Your ear, being relatively exposed, is particularly susceptible. You get more accurate readings from under-the-tongue or armpit readings. Of course, rectal thermometers are the most accurate. I think the different types vary in readings by 1-2 degrees because they're calibrated different to attempt to compensate, but if you come in from outside, your ears are going to be way low.

Have you talked to a doctor about the shivering problem? It could be nothing, but it's probably worth mentioning next time your in the office, just to see if he has any ideas.