Sour milk when defrosting... help!

So... I've been pumping and freezing milk since the first month to get ready for going back to work. The past few weeks I got a nanny a few hours a week to ease into it and we started introducing bottle with breast milk. When I pump and give it to him fresh theres no problem. However the frozen milk, after being heated, tastes and smells awful!!! You know that smell on your clothes a couple of days after it's been stained with breast milk? Exactly that.

I tried diffrrent things like going straight from the freezer to hot water, putting it in the fridge overnight, tried older milk and recent milk... everything is just sour!.

I did some research and apparently it happens to some women and the milk is fine but it just tastes awful and my lo doesn't want it.

Anyone on the same boat here? Anything else I can try?

If I keep it in the fridge is fine but I want to be able to keep it for longer than 24hs...