Number one item is check the pad (as many others have said).

Number two is how was your body doing at the time. If you push yourself too hard, sometimes it is hard for the core body temps to come back up even if you refuel. I would guess that this is the thing most do poorly, that is monitoring yourself physically. Sometimes you just have to stop and get yourself "back in order" again before continuing on.

sK

P.S. A number three; I am a big guy and I have noticed that my WM ultralight (rated to 20 degrees) works great, but I do have to be a little careful about loft distribution otherwise I get cold spots. My heavier bags never have this problem.