What could be the reason that my Toilet is leaking?

Check the second floor toilet for water coming from underneath onto the floor when the toilet is flushed. I would assume that the wax ring on the bottom of the toilet has failed. When the toilet is flushed water bypasses the wax ring and flows down to the first floor ceiling. Replacing the wax ring should solve the problem.

More reasons than I can count. You need to have this looked at and fixed ASAP! Water can cause mold issues, rot floorboards and joists, and compromise the structure itself. That will cost much more than the price of fixing the problem of a leaky toilet now. Good luck!

Toilets have several joints which with time can fail. The first as noted above is the wax ring, the second as also noted above is the rubber ring fitting between the tank and the bowl. The third is the fitting between the water supply line(there are two of these-one at each end) and the tank. While you have your toilet apart go ahead and replace all of them. They are fairly simple to replace and at the same time replace the ball/float or can on a stick flushing mechanism. All together it will cost you about $30 at one of the big box stores. And about 2 hours of your time.

Brenda if the tank was leaking it would be noticeable. Water on the floor. If toilet bolts are loose then the wax ring is compromised. Tightening them doesn't solve the wax ring problem. Toilet bolts just hold it down to the floor and prevent it from moving. There is no rubber gasket in the tank. There is a rubber gasket between the tank and toilet, but it rarely goes bad.