Can A Rho-GAM Shot Cause Miscarriage?

I had always been told I was O+, had 2 healthy children and then a miscarriage. During the miscarriage I was told I was O-, and therefore needed a shot, which I received. During my 3rd viable pregnancy I was retested, and assured I was +. After delivery, I was typed again, and told I was -, and again received a shot. I have since had another mis. This has all been very confusing to me, and I am so worried that my miscarriages could have been avoided.