Due date still.changing - freaking out

My first scan was 7.31 and i was suppose to be 7w1 day but i measured 6w. I went in today exactly 7 days later and only measuring 6w4d. My periods are never later than 2 days so im very regular. Theyve seen a heartbeat both times. Todays was in normal range but on the lower end. This is my 3rd pregnancy and would be my 3rd baby. I feel like im doing something wrong or not doing somthing thats not letting this baby grow. I should be 8w by now and im almost 2w behind. They Put me on pelvic rest for a hematoma possibly from implantation. I just dont know whats happening.