What Causes Bleeding Dark Brown Blood While On Birth Control?

Hi, I gave birth to a baby boy on Jan. 14 2010, since then i have not quit bleeding, the doctors here have done 4 ultrasounds and said everything was clean, and put me on birth control a month and a half ago. I started taking 4 pills for 2 days then 3 for 2 days and then2 for 2 days and then 1 ever since. My bleeding had turned to a dark brown and now i am bleeding again with a mixture of bright red and gobs of dark brown. Can you help? i have never went off the pill for the last month and a half.