When does apple cider go bad?

I bought a gallon of pasteurized apple cider a week or so ago so that I could use it in my turkey brine recipe for Christmas. It clearly says on the label "refrigerate for best quality, or after its open." I went to make the brine tonight and when I picked up the gallon of cider I noticed that the jug was semi "bloated" with stuff floating around in the liquid. It looks like pieces of apple, pulp I think..really tiny specs..but still noticeable. I shook it up (unopened) and it seemed to dissipate but there is still a dark reddish-brown "syrup" on the bottom of the jug. When I put the jug down on the counter...still unopened..it was fizzing and getting a foam at the very top..just before the opening would be. Does this mean the cider has gone bad (or has fermented?.but can I still use it?) The brine recipe I'm using calls for a gallon of cider to be boiled with other ingredients, then chilled before use. Is it still safe to use?