What Causes Heavy Menstrual Bleeding?

Hi, I'm a 27 yrs old, female, married 3 months ago, height 170 cm and weight 59 kilo. I always have very heavy bleeding during periods since the very beginning of my menstruation, when I was 11. I have been trying to understand the reason behind this and have also taken medications in the past, but nothing has helped. I am a vegetarian since birth and do regular exercise.