What does black/ dark brown stringy mucous from vagina with abdominal pain mean?

A few possibilities: Pain is your body's early warning system that something's wrong and it shouldn't be ignored. A mucous vaginal discharge with abdominal pain could be anything from a pelvic or vaginal infection, an abnormal pregnancy or just a coincidence of 2 unrelated symptoms. You need to seek medical care for this at your earliest convenience.

Old blood: Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections ; hormonal changes can cause spotting. If there is a concern for pregnancy then do a home pregnancy test . There are many options available to help regulate the menstrual cycle. Schedule a visit.