Hello, Irregular periods is caused by hormonal imbalance (like prolactin, oestrogen, progesterone,) hypothyroidism, reproductive tract infections, PCOS, anemia,fibroids, stress etc, Hormonal imbalance, hypothyroidism, PCOS could cause weight gain you need to be examined first to find the cause, don't worry all can be managed with medicines, and which helps to reduce your weight too please consult a gynecologist.