6 weeks postpartum, had small blood clots yesterday, today started black discharge, looks like blood..is this my postpartum period?

Yes: At 6 weeks postpartum, it's possible that this dark black discharge is old blood left from delivery, or it's possible that it's a period as well. If you are breastfeeding exclusively, then more likely it's lochia left from delivery. If you are not breastfeeding, then the blood could be a period. Most important, it's possible to get pregnant at 6 wks postpartum. Hope that helps!