Measured 5 wks 6 days, no yolk sac

Angel • Our first baby was born 5-5-15. We have a beautiful baby boy named Lukas <3
I had my first u/s yesterday. By my count I was 5 wks 5 days. They counted me as 5 wks 6 days. They said my gestational sac was measuring correctly but they didn't see anything in it. They told me that was normal and I have an appt in 2 wks but I keep seeing other ppl who are in the same week as me and there. Is clearly a yolk sac. Should I be worried?