What Causes Scanty Menstrual Period After Quitting Birth Control?

I have never had a normal period. So I never really keep track of when I have my periods. But I used to be on birth control about 6 months ago, but i quit taking it. From then on out my periods seemed to be normal. but Last month the blood was really dark brown and light. usually my periods are really heavy & last longer. What could cause this?