What Causes Irregular And Abnormal Periods?

I am a 16 year old female, I went to the doctor about a year ago for irregular periods and the doctor told me when I went that everything is normal and give it a year. It has been a year and I am still missing periods, Having long ones, Bleeding heavy, huge clotting and bad cramps, Also I have noticed I am bleeding so heavy I have to switch out pads or tampons every few hours What is wrong?