Lipase is an enzyme that breaks down the fats in your milk to help your baby digest it. Some people have excess lipase, which can make the milk taste "off" sooner.
Milk that tastes bad due to excess lipase is safe for babies to eat, but they may refuse it because of the taste.
(Donated "lipase milk" is often given to tube-fed babies who won't taste it.)
This can happen with refrigerated or frozen milk, but we see it more often in frozen milk because the more time that passes, the likelier we are to see the effects.
To test if lipase is an issue for you, freeze some milk and leave it frozen for 2-3 weeks. If baby will take it, high lipase is likely not a concern for you.
If you find you do have high lipase, I would suggest feeding fresh milk whenever possible, and scalding milk that you need to store. Scalding milk means heating the milk to 180 degrees to inactivate the lipase.