Sex and high risk issues

My dr said sex can't cause a miscarriage, but why do some doctors tell us not to have sex for awhile? I am high risk...lost my first baby at 8 weeks. It was right after we heard the heartbeat and decided it was ok to have sex again. I then had a chemical in april in which we were quite active when I thought I was pregnant. Now I don't want anything to do with sex...worried it can really cause complications. Just wondering it there is any research on it...