Your symptoms may be due to a vaginal infection such as thrush or bacterial vaginosis, or less commonly, a sexually transmitted infection. Breastfeeding-related hormonal changes may cause irregular periods, but they do not usually cause heavy discharge with irritation. The lower abdominal pain may suggest a possible pelvic infection that needs medical assessment. You should see a doctor for a pelvic examination and tests such as vaginal swabs, urine testing, and a pregnancy test to confirm the cause. Avoid self-treatment and seek medical care promptly, especially if the pain or discharge worsens.