We live in a one bd apt. Our gas bill for heat only was 287 last month! Why is it so high?

Our bill for December was 197 and Jan was 287. Why is it so high? We have gas heat and live in a pretty average/small 2nd floor apartment. Why is our gas bill so high and we are still cold!! It doesn't make any sense. Any theories or opinions? We called the gas company and there are no leaks and nothing wrong.