At this age,the commonest cause of vomiting is over feeding. Give only breastfeeding at a gap of at least 2-1/2 to 3 hours, don't wake her up for feeds,give nothing else by mouth,burp her for 10 mins after every feed,lay her on her right side for half an hour after burping. Keep her elevated by placing a small cushion beneath the head and shoulders. If still not improving, consult your paediatrician for physical examination to rule out pyloric stenosis.