What Causes Persistent Vaginal Bleeding?

Hi, I had a m/c 3 months ago. After that I had one normal period, 2 weeks after the m/c and since then I have been bleeding almost every 7-10 days. The flow is rather mild and it lasts for up to 7-8days. The discharge is brown with some clots. It's been over 3 months since the m/c and im still bleeding. Is this normal? My GP did all sorts of blood tests, and they all came back normal, no hormonal imbalance, no infection, nothing. So why am I still belleding? I am still waiting for my ultrasound but this constant bleeding really worries me. Has anyone been in a similar situatuion?