My toilet is periodically re-filling, not very often but let's say every hour or two for a few seconds. Is that normal or should it be doing it only when flushed. My gut feeling is maybe because water is seeping through the stopper and the water level would go low, so the fill-pipe (I don't know what's that called in the toilet) would sense it and start running more water in.

 

Normal is only when flushed - you have water leaking from the tank into the bowl.

Ever use bleach tablets in the tank?

 

No, it is not normal. If you don't see any external leaks then the flapper valve is probably leaking. A fix might entail just replacing the flapper or it could entail a whole new flush valve assembly.

 

No, what do bleach tablets do?

 

Thanks, I hope it's just replacing the flapper. All this started when I just shut off the water so I can replace my bathroom sink, and when I turned on the water again, it seemed to start acting up. Hope it's just the flapper, though a new flush valve assembly is not too hard to replace, just a pain to do.

 

Bleach degrades the rubber on the flapper valve, causing it to leak. Common for someone to have a leak because they've been using them - we actually changed our leases about 10 years ago to disallow their use after some tenants had water bills several times normal amount due to leaking flapper valves.

 

Almost all "automatic" bowl cleaners that sit inside the tank contain a chlorine compound. This will eventually rot the rubber of the flapper.

Also check that the water level is not too close to the top of the overflow tube, should be about an inch lower. Make sure the refill tubing is not shoved down into the overflow tube, it should be clipped to the tube in a manner that the refill tube is very close to the upper edge of the overflow tube.

 

Furd has a good point.
One of my toilets was doing what the OP describes because of the overflow tube.

(It also had a leaking flapper because of the abuse of the hardwater.)

 

let me tell you i had a periodic refiller at one of my apartments recently and went through 2 flappers! Still refillling, then rebuilt the entire flush assembly with NEW gaskets thinking it was leaking into the bowl from there.... STILL REFILLING!
Was about to buy a new toilet thinking there might be some odd hair line crack causing me issues but then decided the only thing left was the fill valve, but in all my years until now I never had a fill valve cause periodic refills.
for some reason the fill tube that goes into the center of the drain valve would trickle water down it making it appear in the bowl like a leaking flapper BUT only when in there. As soon as you would pull it out to check it it would stop dripping like a siphon was broken. Finally figure it out by just holding it down next to the drain for a while after flushing and you could see it would start trickling then the fill valve would make a hiss like it was filling.
change your flapper and if the fill vavle is more than 2 years or so old just change it for like 6$. It worth it and might save you a headache