How do I know? My first miscarriage

Alysia
Today I was told I've had a missed miscarriage.  I'm supposed to be 8 weeks, I was at the office last week when we first saw my baby's little heartbeat.  Through ultrasound they were looking for a heartbeat and they  couldn't find it and my baby measured 7 weeks. So they said I lost it last week.    I've chosen to wait and let things happen naturally.  But how long will this take.  I've not had any bleeding or cramping.  Could they have been wrong? I'm just so confused! Has this happened to anyone else?