Sour Milk?

I just started back to work and have started thawing out milk from my stash and have found some of it is sour and some is fine. Some of it is fine until I warm up a bottle and then it smells sour. I stored some of it Medela containers and some in the bags. It never defrosted and refroze and it was fresh when I froze it. Also it's only from Oct. Any one else have this problem? Any advice/suggestions to keep this from happening? TIA I also posted on the bf board.