What causes dark brown menstruation?

Slightly graphic but no pics. What causes the dark brown sticky/clumpy stuff during AF? I typically have a bright red period, this time AF was 6 days late, I had a positive OPK? Is it "old" so it turns brown, is it even a healthy egg if it's that color? I've had it be faint and brown around the last day, but never start out this way.