Ok I just looked it up on the ABA site and they say:

Nausea during the let down does appear in the published breastfeeding literature though most of the information is from case histories and unpublished anecdotal information (Lactation Resource Centre 2002). From unpublished information it appears to be related to the following: medications particularly anti-depressant medication; excessive fluid intake; not enough fluid intake; low blood sugar: low blood pressure; thrush in gastrointestinal tract; low grade urinary tract infection; eating disorder; tiredness; abuse memories; hunger.
Of all these causes hunger is the one backed up by research. This research shows increased gastrointestinal activity when the let down occurs (Widstrom et al 1988, Uvnas-Moberg 1989, Uvnas-Moberg 1989). The production of gastrin (a polypeptide hormone) occurs as part of a protective effect to ensure adequate energy supply for milk production. This subsequent increase in energy uptake can then lead to nausea. If there are other factors involved this could exacerbate the effect of the extra gastrointestinal activity. Anecdotal suggestions that may help are: high carbohydrate snack before a breastfeed (Lawrence & Lawrence 1999) and sea bands used for motion sickness. Mothers reported that the problem eased over time, varying from a few weeks to some months.