Why Is My Newborn Preterm Baby Vomiting After Feeding Dexolac?

Hi doctor, I delivered baby girl with weight 1.870 kg at 35 weeks and 2 days (preterm baby), now baby weight increased to 2.5 kg and baby age is 33 days .Doctor prescribed us dexolac but from last two days on wards she is vomiting seems it’s not digesting ,why ? in the first month we didn’t face any problem like this and also can you tell which is best to use dexolac or nan pro