I would suggest ti check for allergies or intolersnce to cows milk protein. Eaven though baby might be on breast milk the proteins can go through your milk. Mind you this is the worst case scenario since im going through this my own, keep an eye on his poop- very important, also other indicators like eccessive gas or colics or eaven rashes or eczema, if the poo is stringy or mucousy its another indicator of intollerance/ allergy. In any case you can read about that and ask your pediatritian for assistance. Hope this helps.