My car is using a lot of fuel?

So I drive a 2001 honda accord V6 and It has 105,000 miles on it and has been well maintained. I usually just use it to drive to school and maybe a few errands and I usually only have to fill up every two weeks but I filled up on Thursday and today's Monday and I'm about at a half tank somehow, what's the deal with this.

Not necessarily the problem, but the first thing I think of when fuel consumption suddenly skyrockets is a thermostat stuck open. It can cut fuel economy in half.

It is easy to check for. When the engine is cool, start it and feel the upper radiator hose as it idles. It should stay cool to slightly warm as the engine warms up, then rather rapidly become too hot to grab when the engine reaches operating temperature. If it warms up as the engine does you will save money by replacing the thermostat.

Check for a vacuum leak.