Dark Brown/Black blood with horrible cramps at start of 8 days late period?!

Hi ladies, I was hoping this month was the month we got that BFP as I was 8 days late. Anyway lots of BFNs and today my AF has started but it very dark brown nearly black, blood. I never get cramps either and they've had me up all night and in bed most of today. Any idea what causes this? My last period was fine, normal red blood but last 1 day longer than usual. This is completely out of whack for my body 🤔