Don't know how many people know this, but it's a fairly common practice for sewage rates to be determined based on water usage during winter months. That is, seeing as how you probably aren't running your sprinklers too much, a fairly easy characterization of how much water actually goes down your pipes can be found in winter months.
If you are filling a pool during this time, it might misrepresent your actual water usage and cause you to have a whole years worth of higher sewage bill.
You might check with your biller as a they are often willing to exclude a particular month or change the months that they look at for such an exception. Similar case for a slow leak that built up a large water bill.
This isnt a HUGE difference, but with say 20k gallons of extra usage, you might be able to pay for your pool beer for the year by making that phone call (or maybe not, I guess I don't know how much you drink).