I thought my DD was allergic to milk because she reacted when I ate it. She "grew out of it." Shay had bloody diarrhea when I ate dairy, so off I went again. He has no problem when Sara eats it. Soren, same blood and mucous if I eat dairy and other issues if I eat wheat (green slimy poop, tons of eczema) but no issues when Sara eats those foods. Anyway, it still makes me worry about testing him on dairy. He had a cookie once with a smidge of dairy that messed him up bad, so I'm holding off. He has had nut butters from about 6 months without issue, tho.
I've read that breastmilk reactions from babies mean that both mama and baby have sensitivities to those things. Or, in the case of younger babies, that mama has issues with those things and the little babies shouldn't have those things because of their immature guts. So if mama can't properly digest dairy, the whole undigested protein passes through the milk to the baby. This is why little babies seem to "grow out of it" - because their guts mature. And also why one mama nursing can eat those things, but the other can't, with the same baby.
But, I don't have links or studies or anything to prove this. This is just a case of "I read that somewhere at some time." Curious to hear what others think about this.