I would go and get it checked out - even if you have to see a different doctor.
Something similar happened to me, bleeding on and off and then more closts (big ones actually) and I went back to the OB at about 5 weeks and it turned out I had an infection. I didn't have any other symptoms and a course of ABs cleared it up (no retained placenta).
I'm sure the length of time that women bleed for varies a great deal, but the clots might be a worry.