Implantation or period??

My period isn't due until Sat but I just noticed heavy, red spotting. We did BD during my fertile window about 10 days ago. I don't know if this is implantation, my period, or miscarriage (if that's possible). I've been regular with my cycles since I got my IUD out in October, but I wonder if it's possible that's what it is? I've also been getting negative pregnancy tests but it's still 6 days before my period. Anyone ideas what's going on??