What could be the cause of my recent high water bills?

You may want to check to see if a toilet is leaking. Check the water level in the tank to see if water is overflowing into the overflow pipe. This is the pipe in the middle of the tank that has a small tube connected to it. If water is running into the overflow pipe, adjust the fill valve to stop the flow approximately 1 inch below the top of the overflow tube or to the water level mark stamped on the side of the tank. Periodic maintenance by a plumbing professional will ensure proper operation.