Help with frozen BM

I thawed out some BM from about a month ago (which was about 1 week PP) and I gave it to baby this AM and he gaged and ended up having reflux the rest of the day (we’re moving to all formula soon so the rest of the day has been that). I didn’t think much of it but just tried it and it tastes absolutely terrible. when I tried it recently (fresh) it’s always normal and sweet. Is this from freezing it? I have so much in the freezer now I’m sad I may need to toss it all 😔