Missed miscarriage misdiagnosed?!

I was pregnant with our first baby and went in for our 8 week appointment and the doctor said the heartbeat seemed to have stopped at 7 weeks 4 days. She prescribed me norco just in case I'd pass it and then have an appointment on Monday to follow-up. I'm just curios what peoples experiences are with missed miscarriages. Has anyone had a misdiagnosis? It's only 3 days off between 7+4 and 8 weeks, could there be a mistake? Trying to stay positive, but are expecting the worst. Baby Dust to those ttc and Sticky Beans to everyone with a Bfp. :)