Low rising HCG and very tiny sac

Hi all, I am 6w 5 days today and had my 1st scan today. I went through Frozen transfer IVF so embryo is definitely inside. On my scan today they found it difficult to see the sac and were not 100% sure if it was a sac. They said it could be a cyst with a cavity sometimes. I have been asked to go for a blood test and scan again on Sunday. My today’s HCG was 1644. I am super worried if it will be ectopic pregnancy. My scans are usually super painful due to endometriosis. This is so stressful! Have any of you gone through situation like this? I am not sure what will happen from here on.