My Baby Is 14 Month Old She Is Pettite N

My baby is 14 month old she is pettite n kinda small the doctors keep telling me to give her whole milk but she is very grpuchy n bloated at night n she throws up at times a very bad smelling throw up. I changed to lactate milk but same result i know all doctors want babys to be on whole milk but its not helping my baby plus she rerly poops i have to guve her alot of miralax and still its tough. She took Nido for about a month before i put her on regular whole milk n she pooped good and everything im thinking of switching her agein but im not sure if that would damage her morw