It is common for mineral deposits to form on the thermostat inside the tank. Use a meat thermometer or digital thermometer to verify that it is too high before replacing the thermostat. Generally 120 F is a good setting. If it reads higher than it is set for, replace it. Step by step directions for replacing a thermostat are on my blog: realgreenplumbing.com. I've done oodles of them in my career. Good Luck!
Maybe the thermostat on your hot water heater got turned up accidentally. Was anybody by your hot water heater lately that could have done it. You can turn it down yourself. It's usually a red button on the side of the hot water heater.