Breast milk? Can you still produce enough breast milk? Don't switch on formula yet. If bm is not an option, try several times before you give up. Baby need some time to adjust to the new formula. Be patient, keep on trying.
Something else? Nutramigen tastes very different than breast milk or other formulas. Does your baby have to eat it? Is it for colic? Could you try something else, such as goid start or gentlease? Ask your pediatrician.