What Causes Heavy Vaginal Bleeding With Clots?

I have a paragard iud. My cycles are always 28 days. This last one was 2 weeks late. At first I just spotted off and on for a few days. In the last couple of hours I have started bleeding very heavily and have passed 7-10 large clots. ( 2-3 fingers wide and long) is it possible that I was pregnant and am now miscarrying?