Hello. According to your history it seems your period are quite regular. But the spotting on 19th day of cycle is not usual. You have to take care of few things.

Black vaginal discharge may look alarming, but it isn’t always a reason for concern. You may see this color throughout your cycle, usually around the time of your regular menstrual period. When blood takes extra time to exit the uterus, it oxidizes. This can cause it to appear a shade of brown to dark brown or black in color. It may even resemble coffee grounds. Black discharge may be a part of your menstrual cycle and require no special treatment. When the discharge is heavy and accompanied by other symptoms, like fever, pain, or a bad odor, you should visit a gynecologist to rule out the following conditions such as stuck or forgotten objects, pelvic inflammatory disease, other infections, implantation, missed miscarriage, retained menses, etc. If the problem persists or worsens kindly visit the nearest gynecologist at the earliest.