I've read that some women have excessive lipase in their milk, which can cause the milk to taste bitter/smell funny once it's been chilled or frozen. Some people say that scalding the milk on the stove and letting it come back to room temperature before chilling/freezing helps a lot. Other say their LO's didn't notice a difference, and mom only noticed it because she could smell a difference once the milk was defrosted. Also remember that breastmilk can remain stable at room temperature for several hours. So if you are pumping for the day and not for long term storage you may want to leave some out on the counter if you don't keep your house too warm.
Re: LO doesn't like frozen milk