Sounds like a Poo-Dam. Your tank was dry and the poo and paper contacts the dry surface with insufficient water flow to move the poo and paper itself to the valve. Some of the waste sticks to the tank's bottom and hardens. Soon you have a poo ridge that keeps building and building because the water flow just isn't enough to slide things down to the drain.
Solution is a good tank clean and ongoing use of sufficient water each time to ensure things are working as designed to work. If the weather is too cold to properly use the necessary water, you may want to consider investing in a black tank heater.