i came home to abnormally low water levels in two separate toilets on the first floor of my home. after flushing one of the toilets (with no solid matter in the toilet), the water filled to the brim and started to overflow until I turned off the valve manually. several hours later, the water level in that bowl is back down to an abnormally low level. i'm too afraid to even flush the second toilet for fear that an overflow will happen again. when i went upstairs, the water level in that toilet seemed normal. what is going on?