We went from 9 therms in Oct. 08 to 14 therms in Oct. 09 and from 11 therms in Nov. 08 to 18 therms in Nov. 09.
That's a pretty large increase without any assignable cause. In two months of comparison your gas use increased from 20 therms to 32 therms, a 60% increase. If your habits/vacation absence truly are unchanged and the weather has been comparable then it appears to be a significant difference. It might not be a problem, but I would check the unit daily for awhile to make sure it hasn't sprung a leak.
In 17 years has the unit ever been serviced or blown down?