Have you considered that it is your heating pad? I have a very limited knowledge of such things, but my understanding is that in significant temperature drops, heating elements can over heat themselves just trying to maintain their original function.
This one has a thermostat control and I don't know if it can exceed that control on its own...but I don't know about such things either. I know when it was the same ambient temps in the room in the day as it was at night that the nest still fluctuated in this manner...long about evening when the room starts to cool and long about morning when it starts to warm, both times it's near 50* in the room, the nest will warm up in the night but will cool down in the daylight and the ambient temps in the room are the same.
It's a puzzlement.