I think keeping a food journal is a great tool for pinpointing which foods result in your LO's discomfort.
I don't know how quickly a digested food shows up as an allergin in your milk.
since foods we eat are digested before our bloodstream can carry the nutrients to our organs I don't think there is a biologically identifiable molecule of any offending food in mother's breastmilk.
Although I did recently read that certain proteins pass into the breastmilk, like cows milk and other foods are irritable to a mom like eggs, corn and soy but they don't pass into the breastmilk.
DD#1 July 1986 VB
DD#2 April 1988 c/sec
DS#3 April 1990 VBAC
DS#4 June 1993 VB
DD#5 April 2001 c/sec
BTDT scars and stretchmarks,: wrinkles and grey hair