Gestational Sac but no fetus....

So according to my glow app I am 8 weeks 5 days. I went in for an ultrasound today and they thought I was a week further due to when my last period was. When I had the ultrasound they had to complete an interior one. During this there was a gestational sac but no yolk sac or fetal pole. My doc took my hcg levels and they are about 41,000. She prepped me for a miscarriage to start taking place but am going back in at 48 hours to determine if my hcg levels are decreasing. I have no idea how to feel. I want to stay positive but it is hard to. I am still feeling a little nauseous but I’m not sure if that is just from these emotions.

Also side note, this is my third pregnancy. My 2nd pregnancy I also had a small gestational she with my son. Has anyone else had this experience? Should I just get mentally prepared for a miscarriage to start happening?