Advice with feeding Nervous

So my daughter willow her feeding is all off it worries me sometimes she will eat a ounce and a half rarely 2 but then she will only do half an ounce at her 3 hour mark and will not take anymore for another 3 hours no matter what I try she is a week and 2 days old what should I do could it be the bottle?? I use Playtex baby the curved vent bottles I have used the Medela but same results she is growing but she doesn't reach her mark for formula for the day she also rarely will Burp no matter how long I go at it