I bought some bottled kefir for my son last week.....two days ago, I poured him some, and he didn't finish it, so I covered it and put it in the fridge. I just pulled it out and gave it to him to drink, and he brought me the cup to ask me for something else to drink. The cup REEKS like vinegar. Does that mean the kefir went bad? Is it going to make him sick?