A small percentage of breastfeeding mothers experience feelings of depression (or anxiety, homesickness, agitation or anger) beginning immediately before their milk lets down. This is called Dysphoric Milk Ejection Reflex, or D-MER. According to D-MER.org, “Dysphoric Milk Ejection Reflex is a condition affecting lactating women that is characterized by an abrupt dysphoria, or negative emotions, that occur just before milk release and continuing not more than a few minutes.” This is a physiological response (not a psychological response) that appears to be tied to a sudden decrease in the brain chemical dopamine immediately before milk let-down.
Following are some resources for more information:
D-MER.org is a website devoted to information about Dysphoric Milk Ejection Reflex
Dysphoric Milk Ejection Reflex (D-MER) from the Australian Breastfeeding Association
Heise AM, Wiessinger D. Dysphoric milk ejection reflex: A case report. Int Breastfeed J. 2011 Jun 6;6(1):6.
Cox S. A case of dysphoric milk ejection reflex (D-MER). Breastfeed Rev. 2010 Mar;18(1):16-8.