The flapper is warped.

You need to run a test.

1. Flush the toilet.
2. Make sure the water level in the tank is at its proper height, which should be approximately 1/2 inch below the top of the overflow tube.
3. Using a pencil, draw a line where the water level is, so you have a starting point.
4. Shut off your water supply. Your water supply can be found on the left side of the toilet coming off the wall or sometimes off the floor. Water is going to continue leak out.
5. Wait for the water to stop leaking. This happens when the water level reaches the location of the leak. You must allow 24 hours to do this.
6. Return to this site and select "I ran the Leaking Tank test and I am ready to continue."