Dark brown/blackish discharge

So it's been about 2 weeks since my period. I had sex a week ago. Since having sex I've had brown/black discharge. It started as normal red blood color and is getting darker every day.

I know it's old blood and the oxidizat process. But why is it happening? Now? I've never had this before. I don't have the implant or any other bc, I don't have any other symptoms, no smell or anything. It just looks...nasty.

Any idea what this could be?