What is wrong with my 03 Honda civic EX?

So I've replace new belts, spark plugs, and a new thermostat and after that a few weeks later today my car shows on the gauge that the temp. Is really high and for some reason my gas gauge also went down and my tank was almost full when i was driving it the day before. It doesn't make sense to me someone please help?

When is the temperature showing high? When you first start it for the day? After driving around town? On the highway?

Have you checked your oil for coolant and coolant for oil?