first of all i would advise you to give breast milk the child till it is 6 months as the babys gut cannot handle anything other than mothers milk. you can store the milk in a clean container and it can be kept for upto 12 hours and there is no need to boil the milk if kept in a clean container.
lactogen is safe and contains all important nutrients. but if your baby has abdominal distension vomiting
and loose stools either you are over feeding or it could be a condition called lactose in tolerance to to the absence of the enzyme lactase.
i would advise you to consult a pediatrician
i hope i have answered your doubt.
take care and wishing your baby a speedy recovery