What about last year? Are you new to this place? Is 229 therms expected for this time of year? How many sqft is your place? How old is your heater? What temperature do you set the heater at?

I'd also ask for an investigation, but I'd also assume it's something weird in your house, because it's always easier to actually diagnose the problem if you assume that first.

Do you have the ability to look at the numbers at your meter? Look at the numbers. For me they're measured in hundreds cubic feet (ccf). 1 ccf of natural gas = 1.036 therm. You can visually see if the readings on the meter add up.