Tank leaks (on to floor) with new flapper--why?

Today, I replaced a worn flapper (due to extended running into the bowl after flushing) on a Cisa/Briggs 2241 toilet. I only replaced the $5 flapper/chain (Toiletmaster) part--not the entire flush valve/gasket/overflow tube unit.

Now, I'm noticing dripping water on the floor coming from the tank. I can feel wetness between tank base and bowl near the tank-to-bowl gasket. It appears to be a steady drip and not increased with flushing, but I'm not confident of that.

I had no leaking from the tank to the floor prior to replacing the flapper. Because I didn't replace or fiddle with the tank bolts, ballcock, or flush valve/gasket assembly, I'm confused as to what I possibly did.

Is it possible the gasket is just so old that, by disturbing anything on the flush valve, the gasket failed? Any insight would be helpful, because this one makes no sense to me. If it's worth anything, it appears the Briggs 2241 requires some special parts--which i didn't use for the flapper.

You answered your own question. Most likely you bumped the tank and disturbed the old tank to bowl seal or a loose tank bolt.

Thanks - another quick question: if I did disturb the old tank-to-bowl seal, would that leak even without flushing? I was under the impression it would leak from that point only after a flush.