My period is black...

For the full extent of my period it was black in color and I had bad cramps. Then once it ended, it ended for about 18 hours then came back! But it came back bright red and very heavy for only 4 hours or so. I have been on the same birth control for the past 4 years and my periods have always been consistent in color without any cramps. should I be concerned about it being black? The fact that it came back? Im so confused! Help!