Empty sac

At 6weeks 1day my hcg levels were 14,000 my doctor decided to do a ultrasound 3days later we saw a sac but nothing in it.. Then wanted to re test my hcg levels that same day my level was 30,000 at 6weeks 4days.. I have to have another ultrasound in 1week. Has anyone else experienced high hcg levels and not been able to see anything in the sac