My fresh expressed breast milk tasted AWFUL WHY?!

I just pumped about 20 min ago and transfer the milk to his bottle and the rest i stored in the fridge. He starts drinking it and starts crying and i smell the milk and it smells expired so i go in and taste the milk in his bottle and i literally gagged ! I went to the fridge to try the other milk that i had pumped with the milk in his bottle and it did not taste like that . Whyy ?! It tasted so bad i had to throw it away as he wasn’t even drinking it and i didn’t even want him to drink it . What could have caused it to taste so bad?? I left it room temperature for max 30 min maybe so it couldn’t have gone bad?! What the heck was it something i ate?!