[b]Since you admit that the engine got hot, there are two things to check for a sudden drop in fuel mileage. The thermostat and the coolant temp sensor.
If the thermostat body got distorted from running the engine hot, the thermostat may be stuck open. When this happens, the engine isn't able to come up to full operating temperature.
The coolant temp sensor for the computer. If this sensor was damaged while running the engine hot, it may not be working correctly. And could be telling the computer that the engine doesn't come up to operating temperature.
I've seen either of these components fail after an engine has been run hot or overheated.