Most likely gastroesophageal reflux...just a jargon that says food return from the stomach to the oesophagus after feed instead of staying down. For such babies, you need to keep them upright for up to 30 minutes or more after each feed...so it stays down.....You can keep them in the car seat put in an upright position if you are too tired or busy to carry them that long...others can help too....but if it so bad and not allowing the baby to gain weight....please see your paediatricians for further management at that point. Best wishes