There may be several reasons for this:  a new rate change, outside water use such as for lawn care or pools, or an internal water leak such as a running toilet or dripping faucet.  The Public Works Office has free toilet leak detection dye tabs available to all residents, or you can place a few drops of food coloring into the toilet tank and wait to see if any color leaches into the bowl.  If the dye reaches the bowl on its own without any flushing, then you have a toilet leak which, more often than not, is an affordable fix such as a flapper replacement.