The obvious answer is a water leak. But, water leaks can be hard to find. 

Running toilets are the most frequent cause. Many times you might not even realize your toilet is running as the leak in the tank seal is so small that it's not noticeable. An easy way to tell is to put a couple of drops of food coloring into the tank and then wait an hour to see if the water in the bowl turns the same color as the food coloring you put in the upper tank. If it does, then you have a leak.

The next most common leak is a dripping faucet. You might not think a slightly dripping faucet is a big deal, but it does add up over time so don't put off fixing it.

After that the next most common leak is the flexible supply lines under each sink in the home. Periodically check below each sink to make sure there is no water or moisture under the sink. Not only will a water leak cost you money with the water bill, but it will also cost you money if the wood cabinet is damaged or mold starts to grow. Even as a tenant you are responsible for any damage to the wood cabinets.

Lastly, it's possible that there is a leak in the pipes in the walls, under the home, or even in the ground with the main line leading to the home. An easy way to tell is to turn off all the water in the home and go take a look at the water meter to make sure it is not spinning. 

First, make sure you find the water leak and stop it. If you are not able to fix it yourself, then contact your property manager if you are a tenant or call a plumber if you are the home owner. One of the nice things about renting a home is that the cost of the repair is taken care of by the owner in most cases.

After the leak has stopped, you can contact the water company to ask them for a refund of the sewer charges. Usually a water company will not refund the cost of the water as they provided it, but if the water did not go down the sewer line but instead went into the ground as a result of a leak, then they will typically refund the sewer portion of your bill. This would apply to any leak where the water did not go into the sewer. If the leak was  a running toilet, then it did go into the sewer line and thus would not be eligible for a refund. 

Quick action is important since a water company will not refund the cost of the water they provided you. Don't wait until you receive a high water bill, instead make it a habit to check your toilet and sinks for water leaks periodically.