It's time to do some simple maintenance to your hot water heater.
Preventative maintenance will result in energy & cost savings, and will extend the life of your water heater.
Consider hiring a plumbing company to do semi annual service to all of your water systems, including your hot water heater. Many offer plans that do HVAC and plumbing services as a package deal and can be quite affordable when priced that way.
Generally the things a certified plumber will do to maintain your water heater will include the following:
1) Cleaning out your dirty burner chamber because it can cause a fire and can make burning less efficient.
2) Flushing your tank to rid it of any sediment that may have collected at the bottom and is impeding heating efficiency.
3) Testing the thermostat and pressure release valves to make sure they are operating correctly.