Bleeding problem

[lowi]

Hello, im 18, i was suffering from menstrual problem. I dont have missed periods but in so much stressed and tiredness in school and also in my partime job I started to have a blackish bleeding and afterwards a dark brown bleed that lasts for a 3 weeks but its not heavy bleeding. Take note that I was able to had a sex before this problem begun. I also take pregnancy test and it is negative. And still now i am still bleeding lightly.