What Causes Miscarriage At 8 Weeks Of Pregnancy?

HI, I've have 2 children but I've had 3 miscarriages resulting in D and C. Each time it takes about 5-6 weeks for a HCG test to even show up positive and I have lost the pregnancy at 8-10 weeks. I have ultrasounds done to confirm pregnancy and each time there is nothing there, no embryo or sign of one. I often feel around 6-8 weeks like there is a washing machine inside me. I also notice my stomach protrude about a week before I lose the pregnancy. Is there a term for this? I feel like I'm going crazy. Im not trying to get or avoid getting pregnant but this can be very disrupting to life and bad for my body to keep having D and C. I have an IUD in place since november and I am starting to feel the usual symptoms all over again, which I thought was not possible.