It could take several hours to days for some components to be present in your breastmilk. What is the reaction you saw? Gassiness, rash, irritability? "They" say that you really should only be concerned about allergies if they tend to run in your family. The most common sensitivities are dairy, nuts, and some strong spices (the biggest is dairy). There is a component in milk (not lactose, but I can't remember the name of the protein) that does transfer to breastmilk and many babies can't handle it at young ages. I had to pretty much cut out all milk (could still eat cheese) from 2-9months (while breastfeeding). At ~9months I started drinking 1 small glass every 2-3 days, and worked it up to my normal consumption. If you suspect milk, you should avoid dairy for at least 2 weeks before expecting to see full results (no more reaction in baby). Good luck! Oh, and kellymom.com is an excellent resource for breastfeeding mamas!