Baby throws up after bottle feeding.

So my baby is one week old today.  I've been trying to breastfeed on demand but she isnt gaining as much weight as the Dr would like to.. So in addition, I pump and feed her my milk using Dr. Brown bottles.   Everytime I use a bottle, she throws up some of it even after I burp her.. She does not throw up at all after I breastfeed.  Has anyone experienced a similar situation?   Any advide is appreciated.