What Causes Heavy Menstrual Bleeding?

Hello doctor My second delivery was on septmber 2014,i got my first period after two month.first bleeding was heavy and lasted for 5 days..soon after 2 weeks bleeding started and it lasted for 3 days...this time it came early heavily..exactly i had bleeding after one week and again after a week..first day bleeding is there..second day some blackish discharge..is this abnormal doctor?is there a need to consult with gynaecologist?