6 weeks, Sac but no embryo

Just looking for experiences.. I went in for an early ultrasound due to bleeding I had over the weekend and a miscarriage in January. My HCG levels were 25,000 on Friday and yesterday were 56,000. The ultrasound showed a sac that measure 6w5d but there was nothing in the sac. I tracked ovulation and should be exactly 6weeks today. The tech said it’s still early and I go for a repeat ultrasound next week. Any success stories have something similar happen to them?