I cooked up some lobsters a few weeks ago. The next day, i used the shells to make a stock as i've done before (boiling them with mirepox and some herbs, nothing fancy). I tasted the stock, tasted great, into the fridge to cool. Next day, into the freezer. When i took it out yesterday it smelled off and some of it seemed almost defrosted, kind of oily almost. I tasted it and it was like licking death, tasted like nothing but vinegar so i tossed it right out. How can this happen in the freezer if it tasted fine when it went in? Nothing has defrosted in my freezer (I used some chicken stock last night that was fine) and i've used the same technique a few times before so what could have happened?