Postpartum bleeding (6weeks) ?!?!!

I had stopped bleeding at a out 2 weeks with some sporting but completely stopped at 3 weeks. It's now been 6 weeks since I've given birth, no stitches, and I started bleeding extremely dark blood today... could it be my period even though I'm exclusively breastfeeding? ? With my first my first period didn't happen for a few months after. I thought breastfeeding would slower the return of period... has this happened to anyone else?

Also, my 6 week appt. Is tomorrow, will they have to reschedule that exam? ??ty in advance