A thick menstrual period is a normal reaction because it contains many components, not only the endometrium and cervical mucus, but also shedding cells and vaginal secretions. When combined together, they can feel sticky, but they also contain anticoagulant tissues that do not normally cause clots. If there is a clot, it may be a sign of a menstrual disorder or other abnormalities that need to be treated. Normal menstruation is dark red in color and sticky without clots. If black or purple menstruation or clotting of menstrual blood occurs, this is an abnormal reaction and should be considered as a disease, such as endometritis, cervical tumor, and other problems. Specific tests are needed to diagnose the type of disease and to treat it.