Son is having fever, not clear why, watery stool and vomiting + teeth …

Son is having fever, not clear why, watery stool and vomiting + teeth coming out. So he started refusing breast milk, sucks for a minute and throws up like he's afraid. Breast has to be expressed when full, is this from refusal of breast milk or because of illness? By the way, a couple days ago were first critical days after childbirth, since then it seems like it began. At first he was sucking all night, now he refuses completely ?