What Does Dark Discharge Imply?

i have been on marvelon for the past 3 years and stopped taking it sometime in feb this year. but i started taking it again 3 months ago but decided that i've had enough of the side effects cause by this BCP. this month, i had my period as usual but this time.. i had dark brown/black discharge instead of red blood discharge which has lasted for almost 7 days now. i took a pregnancy test because i thought it was implantation bleeding but it came out negative. what is wrong with me?