Could I be having a miscarriage?

I am about 6 weeks pregnant and for a week I've had brown to pink discharge with deep red blood clots and some tissue (human tissue not the paper kind). My doctor said none of this is anything to worry about since there hasn't been heavy bleeding or cramping. I just feel like something isn't right with this pregnancy. Nothing like this happened during my previous two pregnancies. Someone with experience or knowledge give me an answer? I understand that miscarriages aren't controllable, it would just be nice to know if this is how/where miscarriages start. Thanks