Can anyone help me wrap my head around this ?

So i go to make some cereal yesterday and pour the milk in and its lumpy ? Its a brand new carton smells fresh and it was cold, the fridge was the right temperature aswell , aswell as it was in date , then today i go to make my son some milk with another new carton its cold it smelled completly fresh and was in date by a couple days i poured it in his beaker, it had no chunks nothing , 10 mins later my sister comes up and says shes made a coffee and the SAME milks chunky and gone off ? I go to check my sons bottle and its STARTED TO CURDLE!! I cant think of any normal explanation for this as the fridge is the right temperature the milk was in date and this happened with 2 completly different branded NEW milks ?