7w 6d pregnant with twins. Miscarrying one?

Good morning. So, I found out this past Friday that I'm pregnant with identical twins but baby B is probably not going to make it. Baby A has a good heartbeat of 151 and looks good but baby B is measuring smaller than the other and it's yolk sac is larger than what it should be for the gestation age. I have to have another u/s done this Friday to check the progress and see how things are but I have this feeling that I'm miscarrying baby B. For the past 4 days I've been spotting and have severe lower back pain. I know spotting can be normal early in pregnancy but this is more than spotting but not fully bleeding or clotting. I'm praying that if I am miscarrying that it's baby B that was not going to make it anyways and not both babies. I'm worried because they are identical that baby A will be affected as well. Does anybody know or heard of miscarrying one identical twin and the other being ok? I called my dr and they said there's nothing I can do at this point and I'm on "pelvic rest"--no lifting heavy objects and no sex. I just need some reassurance and some positive thoughts my way. Thanks