breastmilk gone bad ??

Hi mamas! So I have frozen breastmilk from  May so no longer than 3 months in the freezer. I defrosted it and noticed it smelled off so I tasted it and YUCK! I then tried the June and July frozen milk and although they didnt taste quite as bad was still off. Finally I defrosted and tasted the august milk only a few days frozen and it tasted fine . So question is the may-July milk bad or does it just change taste over time ? I thought milk lasted 3-6 months in freezer so I am not sure what to do have a lot of milk and hate to waste it