What Causes Abnormal Vaginal Bleeding And Feeling Of Fatigue?

I have been experiencing abnormal vaginal bleeding. The first bleed was black and brown, sparatic and lasted from January 23rd to February 7th - 16 days long. Then I had a second bleed start February 16th to 24th - 9 days long and it was dark brown and sparatic as well. Then a third started on March 7th. I still have it. I m not on birth control. I had a c section five months ago. I have been experiencing extreme fatigue. So it was 8 days between the first and second bleed then 18 days between the second and third. What could be going on?