I don't know if my defrosted milk is bad?

My baby was born Oct 5th and I froze a ton of milk. I have a deep freezer so I figured at least 6 months the milk should be good for. I defrosted milk that is about 4-5 months old and I do not know if it's sour. I noticed the baby was refusing it and I thought maby it was sour or maybe she was just being fussy. it smells fine I tasted it and it does taste a little bit different then the defrosted milk from 2 weeks ago. it kind of tastes a little bit like it has soap in it.I do not want to dump it all if it is not sour maybe it is supposed to taste like that. It smells fine I would think it would have an odor and it does not.