What Causes Vaginal Bleeding With Clotting And Back Pain?

A few days ago I started having extremely heavy vaginal bleeding. More than I had ever experienced before. At first I thought there were also large clots but they appeared to be more like tissue. This bleeding last for about 10 hours before it was atleast managable. I still have light bleeding and lower back pain. As far as I know Im not pregnant but I am afraid I may have miscarried. I'm not sure what to do.