Definitely sounds like it is fermenting again. It's a good thing it wasn't bottled- you'd have bottle bombs!
When fermentation is finished, and the cider is clear, it's racked off of the lees and into the sorbate/sulfite mixture. Usually it works, but not always. Sorbate doesn't stop fermentation, but inhibits yeast reproduction. It sounds like you had enough yeast to ferment out the juice you used to sweeten. Or, maybe the sorbate was old.
In any case, you could wait until it's completely done, and try again.