Suggest Best Milk For A Premature Baby

Hi im Tranay, 22 yrs old, weighting 125 lbs and im 5 2. My medical history is that i had a a premature girl at 7 months she was 4 lbs and i had a C-S, that was four and a half years ago. Now 3 months ago i had another girl and she was 3.7 lbs then dropped to 3 lbs, i had her at 8 months, natural birth. For some reason she s fussy, crying, has a lot of gas, doctor took her of Enfamil and put her on Lactogen because she was vomiting a lot she s still vomiting a lot with the Lactogen gentle start. What milk would be best for her?