From this page Ekka - "A small percentage of breastfeeding mothers notice an obvious difference in their baby's behavior and/or health when they eat certain foods. Cow?s milk products are the most common problem foods and the only foods conclusively linked by research to fussiness/gassiness in babies, but some babies do react to other foods. Food sensitivities in breastfed babies are not nearly as common as many breastfeeding mothers have been led to think, however.
If a breastfed baby is sensitive to a particular food, then he may be fussy after feedings, cry inconsolably for long periods, or sleep little and wake suddenly with obvious discomfort. There may be a family history of allergies. Other signs of a food allergy may include: rash, hives, eczema, sore bottom, dry skin; wheezing or asthma; congestion or cold-like symptoms; red, itchy eyes; ear infections; irritability, fussiness, colic; intestinal upsets, vomiting, constipation and/or diarrhea, or green stools with mucus or blood.
The severity of a food reaction is generally related to the degree of baby?s sensitivity and to the amount of the problem food that mom ate?the more food eaten and the greater baby?s sensitivity, the more severe the reaction. Food reactions may occur within minutes, but symptoms in breastfed babies more commonly show up 4-24 hours after exposure. If baby has an acute reaction to a new food, or to a food that mom ate a large amount of, then he will probably be back to normal within a couple of hours. If baby is sensitive to a food that mom eats frequently, symptoms may be ongoing."
If you scroll down on that page there's another whole section on sensitivity to cows' milk protein too. It doesn't give a reference to the research, but I'd imagine it's probably google-able.
Unfortunately GPs are notoriously underinformed about most things breastfeeding related .
Will read and respond to rest of posts now.