At 17-years, a WH has been on borrowed time for quite awhile. Not to say that occasionally some don't last longer. Depending on your water quality, it could be trying to heat up the water through a very significant portion of minerals, decreasing the heat transfer efficiency. Might be other reasons, and this sort of thing usually comes on gradually, not rapidly. But, your incoming water temperature may be dropping rapidly as winter approaches, and that will cause it to use more gas. You can get the statistics on temperatures - you might want to compare this year with last. Any teenagers home and taking long showers? Doing more laundry with hot? put in a new soaking tub? You could have a leak...is your house on a slab? Any part of the floor seem warm? Take a look at the water meter and while nobody is using any water, check to see if the meter is moving. You might have a surprise in the water/sewer bill coming (they often go quarterly rather than monthly). If it is moving, shut the supply to the WH off and see if it stops.