Roxanne, some locale with high clorine level will promote deterioration of toilet part rather soon. I would suspect the flapper to be the problem without info tb pointed out. Be sure to clean the surface of seat (flush valve) to remove any possible lime. Also check to make sure the ballcock hose is not stuck inside the overflow tube, as some ballcocks will actually siphon water from tank to the level of the hose stuck inside the overflow tube. One of the best ballcocks known will do this if not used with the supplied airgap retaining clip.