Sleep disruption since starting cows milk.

My little guy is almost 13 months. About 3 weeks ago we replaced all formula with cows milk. Before then he ate cheese, butter, yoghurt, small amounts of cows milk in cereal etc with no problem. He was also an unbelievable night sleeper up to that point. About 4 days into the milk weaning, sleep went to pot. He wakes multiple times a night screaming arching his back, painfully farting, needing comfort and repeat. I’m blaming the milk as it seems to have started around then. Anyone else have this?