Aging tastes off

I bottled my first batch of cider in mid-August. It improved greatly with about a month in the bottle, but by the end of 6 weeks had started to taste off. I used the re-sealable ale bottles with the plastic stoppers that clamp shut to bottles it and kept it in my basement while it aged so it was not exposed to excessive heat. I added campden tablets at the beginning of fermentation but otherwise I had no added chemicals to the apple juice and yeast+yeast nutrient combination.

Is it possible that it kept fermenting or rotted in the bottle? Should I have added something to stop it from doing that?

I'm hoping to bottle the next batch in a week or so and I would like to age this one a bit more.

Did you prime your bottles with sugar or was this still cider? Were the bottles clean and sanitized when you bottled?

I primed them with the cordial standard sugar syrup. 2 parts water to 1 part sugar and heated until it thickens slightly.

I sanitized the bottles by running them through the dishwater sans soap.