Let's face it, lactation IS a stress on a woman's body- lactating women require more nutrition and water than non-lactating women. It's not a "bad stress", but for somebody with underlying health issues, it can "push her over the edge" because of the physical resources used up in milk making. Women with extra fat stores have an easier time with breastfeeding- as they have "reserves" of energy if they don't eat enough one day.
If Angeline Jolie has a hard time keeping weight on to begin with, lactating is going to be an added stress on her body, more so than somebody of "normal weight". So, she did something (lactate) that was physically demanding for several months, along with caring for her children and dealing with her mother's death- and those stresses combined are making her overwhelmed/tired/stressed out/ less healthy than she could be.