What Causes Thick Black Menstrual Bleeding?

sir i m puja 26 yr old in previous month my period comes after 15 days i take all thw twst like hormones and ultra sound no result is come every thing is okbut this month my periods comes after 9 days and blood so thick and black what reason behind this plz tell me any problm s is occur i m not understand what happen with me sir i m puja 26 yr old in previous month my period comes after 15 days .i take all the test like harmones and altra sonund no result is come . every thing is ok.but this month my periods comes after 9 days and blood so thick and black.what reason behind this plz tell me any problm s is occur i m not understand what happen with me?