Sour breast milk from yesterday why?

I’m not feeling well today so I thought I might try a shot of my breast milk I had in the fridge. It was pumped yesterday evening and left out for maybe 3-4 hours. I know it can sit that long out. But it tasted SO BAD, like sour milk. Did I maybe let it sit out too long? I didn’t time it so could have been longer than I thought. I don’t want to give this to my baby either. I tried some fresh breast milk after and it tasted fine, sweet.

Edit: I just remembered I did not seal it in the fridge. It still had my pump parts attached with just a plastic bag over it. Could that be why? Not sealed so spoiled in the fridge?