PSEG is electric and gas. What's the breakdown of each?

What's your heating situation? Radiators or forced air?

December was very cold, and if your thermostat was up, it's possible you burned a ton of fuel for heating.

People don't typically realize, but especially in a leaky old house, setting your thermostat to 68 vs 72 is a huge difference and can save you 20%+ on your gas bill in winter.

There's also an excellent chance you have an "estimated reading" in there. If it was estimated in October/November, you're just paying for October now. If December is estimated, it's possible you got over billed and will have a very low bill for Jan and maybe Feb. A payment plan from PSEG will fix this for you if the lumpiness is causing you difficulties.