Viability ultrasound freaking out

My last period started on 10/30 so I should be 8 weeks but at my viability ultrasound yesterday my sac and the head/rump measured 6 weeks. The doctor basically said it could mean either of 2 things. Either I ovulated later so I'm actually earlier in my pregnancy than expected or this is a miscarriage. He scheduled me for another ultrasound next week so we can see if I measure 7 weeks which means the calculations are off and we'll just readjust the due date, or I will still measure 6 weeks so we'll know there hasn't been any growth in 3 weeks at which point we'll know it's a miscarriage. Has anyone else gone through this before? For anyone who's had a miscarriage before, what physical symptoms should I anticipate if I'm about to pass my baby? I just want to be prepared for any possibility, especially if I'm notat home.