What happened??

So yesterday was my first ultrasound and I went in thinking I was 8 weeks, because we were trying for this one and I knew when my last period was and when I ovulated because I took ovulation tests. Well he went over my uterus and nothing was there. He went over it again slowly but only saw a sac and no fetus. Told me it could be too early or it could be a ectopic pregnancy, so he does an intravaginal ultrasound and tubes are fine. I had 4 positive tests so I know I'm pregnant but could it be possible that implantation happened later than I thought?? I've been crying all day yesterday and night!! And now I'm waiting for my blood work to come back this morning!