angie,
the flapper in the bowl is probably old and hard, warped, etc. an easy way to see if the tank is leaking into the bowl is to sprinkle comet or the like around the dry area of the toilet bowl (above the water level) after the toilet hasn’t been used in a while. if the ajax gets wet, remove your flapper,(the thing the flush lever lifts up to get the water going) and take it to the hardware store for a new one. js