What Causes Irregular Bleeding,breast Tenderness And Tiredness?

We are trying to get pregnant. I am bleeding/spotting black tar-like blood, but when I wipe it is red. Thought it might be implantation bleeding, but this has been going on since Saturday (6 days). My period is due on Saturday (very, very regular 27 day cycle - ovulating on or around day 12). Should I see a doctor about this bleeding. I do have breast tenderness and am tired - PMS symptoms.