I was 12 weeks pregnant.confirmed by drs.i got an ultrasound, no heartbeat, nothing in my uterus or tubes. few days aftr ihad a miscarriage.watswrong?

Hard to say: First trimester is when problems with pregnancy are often identified by the body and miscarried, to avoid futile expenditure of resources and to speed readiness to try again. Many things can cause this. Unless it recurs, it is rarely sought or identified. One thing is that it is almost without exception, nothing that you did/didn't do to cause it.

Its not you: Early miscarriage is usually the result of a genetic abnormality of the embryo that prevents life. I know any miscarriage is difficult but spontaneous miscarriage is the way your body deals with an embryo that cannot survive to term.