The easy answer is yes allergens can pass through milk. That seems like it happened very quickly though. I'm not saying it wasn't the walnuts, but I wouldn't jump right to that assumption either. How old is your baby? Have you noticed any other symptoms of allergy? Tummy discomfort, mucousy stool, congestion? Did the rash linger?
“We are not put on earth for ourselves, but are placed here for each other. If you are there always for others, then in time of need, someone will be there for you.”