What Causes Brown Blood And Clotting Instead Of Normal Period Bleeding?

Hi, I was nearly two weeks late for my period, Last friday i thought i got my period but it was brown blood instead of red it is now thursdays and its still brown but it is extremely like as if it was my normal period; ive have a couple blood clots and they were also brown but i have no cramps. I just wanted to knwo your opinion on this situation. I also took two pregnancy tests and they both came out negative!