Lost another baby at 12 weeks

I don't know why this keeps happening to me. Last year I miscarried naturally at 11w5d. Last night I had an ultrasound at 12w2d and there was no heartbeat. Baby only measured 9 weeks. Why is this happening to me? Why does my body stop growing these little angels around 8 weeks. I don't understand what I'm doing wrong. I have an autoimmune disease. Is my own body attacking the fetus thinking its cancer or something? I hear miscarriages at 5 or 6 weeks is probably chromosomal abnormalities but to grow for two months then just stop??? I don't get it. Myself, my husband, our family and friends are all devasted. Why is this happening???? Has anyone ecperienced this?