Milk refusal

Im a first time mum and my boy just turned one. Last week he had his MMR vaccination and after that he had slowly refuse to drink his milk. Usually he had 4oz of milk but after the jab he had it only for 3oz, then 2oz, then 1oz, then 1/2oz and eventually totally refuse to drink. His food intake remain the same (twice a day) but he’s not drinking any milk. And i had to feed him at night and middle of the night when he’s asleep and drowsy and he will only drink for about 2oz to 3oz. If i dont feed him this way, he totally did not drink any milk. Does anyone faced this problem and anyone knows what happening? And what should i do?