What Causes Bleeding After Periods?

I have had a history with ovarian cysts.. I had a regular period and it ended.
Then two-three days later, I started bleeding again.. I feel awful. How can you bleed for more than three days on a PERIOD ~twice per month.. I have been regular, I am not taking the pill.. I have had the same sex partner for three years.<>Please give me your opinion.
Thanks, Gretchen