What Causes Dark Menstrual Discharge With Severe Abdominal Cramps?

Hi. I get regular periods always on time, and it usually last 5 days, its not due to sunday but i had it last sunday and was bleeding really heavy to wednesday with lots of blood clots and really bad stomach cramps and back pain and a cramp in my left shoulder! it was bright red, and for the last 2 days theres like black sticky stuff comming from my vangia! im still getting cramps but not as bad! can you tell me what this is please? plus i have had head aches aswell