Baby hives after breastmilk

ilanat

Daycare has informed me that LO gets a hive on his arm immediately after his pumped breastmilk bottle.  I haven't noticed this while breastfeeding directly, by the way.  The teacher said I should think about what I am eating.  I am not eating anything new or different and we've been exclusively nursing since he was born, without this issue.  I am also meticulous about cleaning the bottles, nipples and pump parts and have not changed the cleaning agent or process in any way.  Any thoughts?