Black discharge

What is this?? The last time I had sex was about 5 weeks ago and I didn’t use any physical contraception. I’m on the pill but don’t take it perfectly and it’s the one where you stop taking it for 7 days and have a ‘bleed’. I was due to stop taking it last Sunday but I began to bleed on the Wednesday before anyway and I just put it down to stress causing me to bleed early. However, it was never red blood - only dark brown/black substance and lasted about 4 days. I had two of these last week on two consecutive days - both about the size of a PingPong ball. Is this just a bad clot?? Implantation bleeding?? Miscarriage?? Help!