Sudden spitting up can be caused by physiological factors, pneumonia, meningitis, gastrointestinal lesions and other factors. 1. Physiological factors: for example, too much milk during breastfeeding, too many times of breastfeeding, or incorrect posture of the baby after breastfeeding can lead to physiological spitting up, but there are no other clinical symptoms. 2. Pneumonia: usually associated with viral and bacterial infections and other factors, belonging to the common diseases of infants and young children, in an acute attack, will lead to babies spitting up, may also be accompanied by fever, cough, shortness of breath and other symptoms. 3. Meningitis: usually associated with viral and bacterial infections, it can appear suddenly and worsen in a short period of time, which can cause babies to suddenly spit up, and may be accompanied by drowsiness, convulsions, feeding difficulties, fever and other symptoms. 4. Gastrointestinal lesions: for example, gastroesophageal reflux, dyspepsia, intussusception, intestinal obstruction, etc., can lead to spitting up after breastfeeding, accompanied by more than crying, diarrhea, bloating and other symptoms. Babies suddenly spit up, there may be other reasons, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.