Empty sack but high HCG?


I had my bloods done and they came back 3500 so had a scan as I’ve had bad cramps but no bleeding. The sack was there but nothing inside. They want me to re do bloods in 48 hours and then re scan if bloods are higher. I’m only on paper from my last period 4 weeks and 5 days. Is it common for the sack to be empty as they sounded concerned as my hormones levels are 3500 there should be more to see on the scan they are saying? Should I be worried as my minds going crazy!