I had a similar situation last month.
Was gone for a month and when the gas and electric utility bills came in, they were HIGHER than when I'm here?!?
And I took precautions: set furnace to lowest temp available (45), turned off well pump, turned off water heater, turned off power strips (that I have on nearly everything)... basically the only things going were furnace, furnace blower, refrigerator, and answering machine. And I put styrofoam panels over almost all the windows.
What the devil?
Well the cold snap probably had everything to do with it. And/or...
The cat and I are heat emitting creatures. I wonder how much heat we normally put into the system? Also, use of lights add heat...
Maybe my foam panels prevented some solar gain?