For moms who've experienced a low milk supply — including me — the stress from that can create a negative feedback cycle. Stress — about your low milk supply or anything else — can prevent you from getting a letdown (where your body releases milk), and when you have fewer letdowns, your body doesn't register a need to make as much milk, since breastfeeding is regulated by supply and demand.
You can learn more about postpartum depression here and here.