Brown spotting can be due to ovarian problems, hormonal imbalance, stress, anxiety, crashed dieting, use of birth control pills, change in environment, excessive exercise
etc. Heavy bleeding can be due to can be due to hormonal imbalance, fibroids and
polyps (non cancerous growth) in the uterus, endometriosis (deposits of the inner lining of the uterus in the pelvic cavity) etc. In your case, it can be due to hormonal imbalance and should get better in a few weeks.