She may have what we call 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 upright position if you are too tired or busy to carry them that long...others can help too.....sometimes that may not work and .... if it not allowing baby to gain weight....please see your paediatricians for further management at that point