Baby not eating as much and doesn’t burp

Hi Mummies... My baby is currently 6weeks old. She is bottle-fed breastmilk during her feeds. She was able to feed an average of 120, max 130ml. However, I realised she is eating less these days. Can go as low as 70ml. On average about 90ml. Also, after every feed, it’s difficult to get her to burp and most of the time, she doesn’t burp and fell asleep. She has reflux sometimes only. Is there something to be concerned with? Thanks in advance.