What Could Dark Clots During Menstrual Flow?

I finished my regular period 2 wks ago. For the first time yesterday, I started having dark blood/clot bleeding. Not severe but need to use a panty liner. I still have it today when I wipe myself. I have not had intercourse for 6 wks and as I said to you previously, i had a normal period which ended 2 wks ago. What could it be?