In The Truth about Mamahood, Claire writes about depression after she stopped breastfeeding.
The first few days, I felt super sad ending the long era of our nursing relationship. I missed seeing her little face nursing, my little baby happily suckling. Then I started to feel extremely light-headed and got some headaches, which was weird- but then depression hit me like a ton of bricks. I was in my dance class (my first without Uma in a long time) and just started bawling over the loss of my mother.
It does seem that, as she implied in her title, that the intensity of these feelings may be due to the sudden lowering of her oxytocin after weaning. I wonder if it's possible to taper off breastfeeding, so your body wouldn't go through such an abrupt withdrawal?