Help? Should I worry? Ultrasound

I had an ultrasound done today because the doctor said it was necessary because I was having mild cramping, and she estimated I was 5 weeks 3 days, and then the ultrasound Doctor today said that there was no definitive evidence of an ectopic pregnancy and that what we saw in the ultrasound was probably the beginning of the sac? And that the sac was about the size of 4 weeks 3 days. But she gave me paperwork that says she didn't see anything and that the reason for that was 70-80% probability that it's just too early, or that it was a miscarriage 10-20%, or ectopic pregnancy 1-2%. They did a blood test and I have to go back in two days to see if my HCG doubles or not. Do I need to worry? Because I'm really worried. She did a pelvic exam too and said my cervix is closed and that there's no bleeding.