What are the causes of a low body temperature?

I think this is about where the temperature has been taken from. Normally with your standard thermometer it is put either under the tongue or under the armpit. This is your peripheral temperature. Although these are reasonable estimates of what your core body temperature is it's still an area of your body that's exposed to outside temperatures so it's going to be slightly cooler. A more accurate measurement would come from a rectal thermometer. Also, before you get carried away in the hospitals these days they use an infrared-sensitive thermometer that you may have seen, You put it into the ear and it beeps. It'll take your temperature pretty much instantaneously. That's just detecting infrared detected from the ear drum. It's a dark cavernous place, it won't be affected by any of the external infrared because obviously there are infrared rays bouncing about all around us. I don't think she's evolving back into a reptile.