The hormonal upheaval during the postnatal period, especially around the time your period returns (or if you are breastfeeding, around the time the baby starts weaning onto solids) can cause the body symptoms that are very similar to PMS.
Vitamin B6 (also known as Pyridoxine) has been commonly prescribed by GPs for alleviating the symptoms of PMS related mood disorders and as such can also be effective in treating those mood disorders during the postnatal period.
"A recent double-blind, randomized controlled trial in 94 women found that 80mg Pyridoxine taken daily over the course of three cycles was associated with statistically significant reductions in a broad range of PMS symptoms, including moodiness, irritability, forgetfulness, bloating, and, especially, anxiety. The potential effectiveness of vitamin B6 in alleviating the mood-related symptoms of PMS could be due to its role as a cofactor in neurotransmitter biosynthesis. Although vitamin B6 shows promise for alleviating PMS symptoms, more research is needed before drawing firm conclusions." National Institutes of Health - Office of Dietary Supplements
The study also reported that 30-75mg of vitamin B6 also reduced nausea in pregnancy by up to 70% in some cases so if you are suffering with morning sickness (especially if it is affecting your mood), please speak to your GP.