empty gestational sac ? help!!!!

So last week I went in for an early vaginal ultrasound since we have miscarried before he wanted to check things out since I found out I was pregnant at 4 weeks . I was supposed to be 6 weeks 2 days at this appointment , there was a gestational sac that he measured and it was at 6 weeks 3 days but there was no baby and a little tiny blip at the top that he said may be the yolk . I k ow people have had this happen and been off on their weeks but there's not a chance of me being off since my gestational sac is measuring 6 weeks 3 days right ? So that basically says how old the baby should be ? And everyone on here has seen baby and had heart beats by then . I am seriously freaking out and he didn't want to see us for  another two weeks so we go back next Thursday to check if there's a baby . Is it possible the sac is bmeasuring bigger because it's twins and the twins may be not as far along but since they're both in there the sac measures bigger ?? Any and all responses are welcome !!