Help. Am I doing something wrong?

My breastfed baby weighs 10lbs exactly, she was born early at 37 and 1, on April 18th, 5lbs 6oz. Her doctor has constantly got on to me that something is wrong with me because well, I breastfeed. I understand she is small, but as her mother I am only 105lbs, I am small, I was small when I was born. She meets all her milestones like a full term baby, she does GREAT. She is satisfied with 4oz of breastmilk, yet her doctor constantly makes me feel like SHIT for breastfeeding and how small she is.