There was a little ditty-case report on anaphyllaxis occurring in a woman when breastfeeding in the Lancet in 1993-sorry away from references now, but no etiology was explained. I took interest in this because I had a patient at that same time who itched like crazy every time she nursed. I was stumped, and sent her to a dermatologist who believed that she was really reacting to an antibiotic she took immediately postpartum for a UTI. He gave her steroids for about 5-7 days, and she was fine then. She did have hives though too.