Only can see the Water bag via abdomen scan.

Did my first abdomen ultrasound and Vaginal scan today can only see water bag, but no yolk sac or heartbeat. Suppose to be week 6, base on last period count. HCG level done on 18 Jan, only like 1-2 weeks pregnant. Kind of worried as I have miscarriage twice both at 1st tri. Doctor arrange two weeks later for another scan. Any mummies have this experience before? What happen after that? #advicepls