Even doctors tend to get confused about newborn lactose intolerance (so exceedingly rare as to virtually not exist) and CMPA —cows milk protein allergy. Cow’s milk (every mammal milk) is rich in lactose. If your baby drank cows milk formula without issue, your baby handled lactose just fine. If your baby did ok on soy, the problem might have been CMPA. Sometimes, babies react to something in the mom’s diet. If she can figure out what it is, she can stop eating it. I say this not to stir up feelings but to urge any mom dealing with this in the future who wants to keep nursing to find a competent doctor who knows how to manage lactation. Most are not trained well in it. A good IBCLC is worth her weight in gold. A LLL leader can also provide good info. Sometimes it’s as simple as a foremilk-hindmilk issue that’s solved just by keeping the baby on one side longer. (The composition of the milk is different at the beginning of the feed and the end of the feed.) It can be complicated so a truly knowledgeable professional is really important.