Fermentation produces alcohol and CO2, so even at this early point, a small amount of that CO2 is likely to already be disolved in your beer. And yes, for a beer of that gravity, it's definitely not unheard of for the very first stage of fermentation to be complete after 7 days.
However, a caution: just because the yeast has finished converting most or all of the sugar it's going to convert, that doesn't mean the yeast is finished with all its work. You definitely want to give it another week or two (more if you're patient!) to let the yeast finish cleaning up any byproducts (like that cidery flavor you're picking up!).