Suspect water heater is using too much gas
It might be a small leak or several very small leaks.  I had a leak in my Philadelphia home for years and had no clue.  During remodel, I replaced the gas pipes (same appliances), and my gas bill unexpectedly went down by $50 for the summer months.  A bubble test will not do.  Obtain a real gas leak tester, and thoroughly and methodically test each joint.