It could be a delayed reaction or just because he's probably now having so much more milk than before (drinking cow's milk is a lot more than used in cereal, etc). Or it could be unrelated to the milk. If it were me, I'd start by cutting dairy out of his diet only at first. You can probably keep it in your diet since he's been BF and you've been having dairy all along - maybe it's just that he can't tolerate large amounts of dairy directly. Other symptoms of dairy intolerance include constipation, rashes, sleeping problems, gas, etc but often LOs will only have one symptom unless they have a severe intolerance or allergy.