How To Control Bleeding After Having A Miscarriage?

I had a miscarriage 1/16/10. I delivered my baby, but the placenta was still in my uterus so the doctor did a d&c. I am STILL bleeding nearly 3 months later. It is light bleeding, but consistant. I passed a huge chunk (golfball size) this past Sunday. I think it was part of the placenta. Why am I STILL bleeding?! My doctor even put me on birth control to stop the bleeding, and triple dosed me for 3 days, then double dosed me for the following 3 days, then 1 pill per day. She doesn't even seem like she knows why I'm still bleeding.