period problems

Hi I'm 21 and sexually active and I am having period problems like I started on Jan. 8 and ended on the 15th but then I started again on the 23 and it's heavier that most of my normal ones and it isn't stopping it's been 12 days I went to the dr. On Thursday but now they want me to come back this Thursday.... does anyone know why this could be happing... I took three pregnancy test all negative so I don't think a miscarriage... I did have an abortion in July but I don't think that would effect it.