yellow phlegm like vomit of 7days old premature 35W newborn

just want to ask if somebody experience my situation now. my newborn 35weeks premature baby who is 7days old now experience yellow phlegm like vomit. this occurs after hours of feeding. i am so worried right now. her xray abdominal xrays are all normal and no finding were noted. she is still on hospital for further observation.