What Causes Dark Brown Vaginal Diacharge?

hi!i am 20..in the month of march this year i had been to a gynaecologist since i missed for periods for almost 2 months.she prescribed deviry 10 mg for 5 days once a day and then i got my periods.but from the next month on, i havent been bleeding properly..very less blood and more of the dark brown spotting..this repeated for 2 months..what is the problem doctor?