The #1 cause is the check valve between the "city water" and the water pump sticking.
Your fresh water tank is filling from the city water so you should drain some (most) of the water in the tank.
This is usually an easy fix...
1. Turn the city water off then turn the pump on and flush and/or run some water then turn the pump off and go back to city water.
2. Same as #1 but flush while the pump is OFF then turn the city water on.
Hope this works and monitor your fresh water tank. If it has stopped filling then you've fixed your problem.