There are a few reasons that your gas may be burning quicker:
1. You may have a leak in your gas line. If you suspect this, you should call your local gas company immediately to have them come and check it out.
2. Your furnace could be set too high. This will cause your gas to burn through more quickly in order to maintain the temperature setting. Try turning down your furnace a bit and see if that helps stretch out your gas usage.
3. You may have an issue with your pilot light. A pilot light that is set too low will also cause your furnace to use up more gas than necessary. Check the settings on your pilot light and make sure it is where it is supposed to be—usually between 1/4” and 1/2” from the bottom of the burner port opening—and then relight it if necessary following the instructions in your furnace manual.
4. One final possibility is that there could be something wrong with the burners themselves inside the furnace, causing them to use up more fuel than they should be while running properly; however, this problem would likely require professional assistance to fix so unless you are experienced with such things, it’s probably best not to attempt a repair on this one yourself but rather call a heating specialist for help instead