It's possible, but not very common, for infants to be lactose intolerant. It's usually an issue with the milk proteins, not the lactose. Soy milk may be a good alternative since fortified soy milk also has a good amount of fat, calcium and nutrients - just watch to make sure she doesn't have an intolerance to soy as well, since up to 50% of babies who are milk allergic/intolerant have a problem with soy as well. Good luck!