Weaning is linked to post-partum depression, however it's often overlooked by mothers and doctors alike.

A great blog post by A Cup Of Jo outlines its effects:

http://joannagoddard.blogspot.com.au/2012/02/motherhood-depression-and-weaning.html

Here is also a study which details the link between weaning, hormonal changes and depression:

http://www.jfponline.com/pdf/5105/5105JFP_OriginalResearch3.pdf