Aiyooo this is so common la during the newborn phase, memang macam ada air liur yang naik-naik feeling tu especially when they're sleeping atau dalam certain positions. A few of my mum friends had the same thing dengan their babies, it usually settles down sendiri, tapi if your baby is actually vomiting or seems uncomfortable, better check with the paediatrician juga to be safe, because sometimes it bisa be reflux or feeding issues yang need attention.