What Causes Continuous Bleeding After D&C Post Delivery?

My wife delivered a baby girl on 15th November 2010 , and her vaginal bleeding did not stop afterwards. Her doctor told us this is normal till 60 days after delivery of child and we waited. After 60 days vaginal bleeding did not stop. Her doctor asked for sonography. The result showed a particle in vagina about 20*10*10 mm. then a D&C operation was done and this particle was taken out by her doctor. She said that bleeding will stop after D&C. It is now one week after D&C operation but vaginal bleeding has not stopped yet. Her doctor is surprised by this and does not know why this bleeding is not stopping. My wife is getting depressed as bleeding prolongs and worries her. Please help us if you can.