What could be the reason for black clot-like discharge during periods??

Hello doctor,

I am a 24-year-old female, height 5.7 feet, and weight 180 lbs. My period came four days back, usually, it lasts for four days with extreme pain and normal bleeding. But this period had been different, it has been for four straight days of black discharge almost clot-like. I do suffer from endometriosis and I do regularly have unprotected intercourse, as I and my husband have been thinking of having a child. I am concerned about experiencing four days of black discharge and constant pain without any sign of my regular period. What could be the reason for this? Please help.

Thank you.