Here is a possible explanation for the phenomenon Esther described.  At our annual meeting of the BF organization here in Norway four years ago, there was a talk by a physiologist on the long term benefits of BF for mothers and babies, in which she presented info on membrane permeability which may be relevant here.  Some mothers produce milk in which there are measurable amounts of protein molecules from cows' milk, causing symptoms of allergy in the babies.  This is not a normal condition, and the origin is unclear, but may have to do with the mother being exposed to cow's milk as a young infant, when the mucous membrane of her gut was still permeable to large molecules.  This can of course be allergenic in the mother, and if the permeability of her gut to these molecules is permanently altered, so that she continues to have them in her blood every time she drinks a significant amount of milk, she can also pass them on to the baby via her milk, as it seems her body doesn't recognize them as "foreign", but treats them as endogenous molecules on an equal footing with immunoglobulins.  

What I am getting at is this: perhaps the mother has no symptoms normally, when she has these molecules in her blood, but when she nurses, there is a quantum transfer of such molecules into the alveoli and ducts in her breasts, where the receptors may be more reactive (not used to having all that activity there? I am not an allergy specialist!), causing a general allergic response in the mother expressed in the symptoms you describe.
yours¨
Rachel Myr, midwife and breastfeeding counselor, Norway

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