The baby’s diarrhea once breastfed may be caused by lactose intolerance, acute gastroenteritis, dyspepsia and other diseases. 1. Lactose intolerance: the causes of lactose intolerance mainly include congenital lactase gene deficiency, infection, etc. The affected children will have diarrhea, abdominal distension, hyperactive bowel sounds, nausea, vomiting and other symptoms. Breast milk is the main source of lactose for the child, if the baby has lactose intolerance, the baby will have diarrhea as soon as he/she eats breast milk. 2. Acute gastroenteritis: most of the acute gastroenteritis is caused by bacteria, viruses, parasites and other pathogenic infections, as well as drugs, stress and other factors. The main symptoms are diarrhea, abdominal pain, nausea, vomiting, etc., accompanied by fever, headache, myalgia and other symptoms. 3. Dyspepsia: Dyspepsia is caused by a combination of factors, including allergic visceral sensation and gastric dyskinesia. Affected children will have symptoms such as abdominal distension, abdominal pain, diarrhea, constipation, etc., accompanied by insomnia, poor concentration and other symptoms. If your baby has diarrhea after breastfeeding, there may be other reasons, so we suggest that you go to the hospital in time, complete the examination to determine the cause of the disease, and then give targeted treatment or therapy under the guidance of the doctor.