As 2littlegirlz said sometimes the palcenta doesn't come away completely - if left untreated this can cause infection which can lead to infertility.
Also, sometimes the uterus doesn't clamp down and shrink to its normal size leading to haemorraghe. Most doctors give you an injection to speed this up, and also breastfeeding helps the uterus clamp down.
As you are still bleeding after 10weeks, I would advise you see your doctor. Have you not been for any check-ups like your 6week check-up? Did you mention the bleeding? What did they say?
Take care.