2 week old losing weight

Holly

My 2 week old is losing weight still. She is breast feeding and seemingly eating well per doctor and lactation consultant. She does spit up after almost every feed even more so when we try to give her formula. I dont know what to do she was quite large at birth 10 pounds 8 ounces and I had 36 hours of labor so she got a lot of fluids. She is 9 pounds 5 ounces and has been losing weight every day. Anyone ever dealt with this?