Gas bills suck

I have experienced life with a gas heater for over a year and I'm still undecided on how I feel about it. My January gas bill was $300 but the Feb. bill is only $60. How does that happen? The house we are in was built in the 1930s so I didn't expect a whole lot of energy efficiency but previous owners have added double paned windows and new drywall and insulation that have helped. I have also noticed a big change from turning the thermostat down in the winter from 72 degrees to 68 while we are gone during the day and at night when we sleep. But, I am still at a loss for such a dramatic change in the price. Maybe it's the unpredictable weather this year.