No fetal pole at 5 weeks + 6

Had my ultrasound this morning. Based on my LMP they thought I was 7 weeks + 1. I knew I wasn’t because I used OPKS and ovulated late. There was a gestational sac measuring exactly what I thought I actually was which is 5 weeks 6 days. But the sac was empty. Even after doing a trans-vaginal they couldn’t see anything. I’ve lost 3 pregnancies so Im fearing the worst. We also have friends who had an ultrasound at 5 weeks 5 days and heard a heartbeat. So thats not helping. Has anyone seen an empty sac at their 1st ultrasound but it was just too early and when they went back there was a baby? Thanks.