ultrasound dates completely off

Ana
I had my first transvaginal US last week, based off of my LMP I was 6 weeks, they couldn't see anything. I had a follow up transvaginal US today and all they saw was a small sac, dating me at 4 weeks 4 days when I should be at 7 weeks. My hcg level was 2079. I am now terrified. I should add that I took a pregnancy test at day 34 (had taken 2 prior since I was late all negative) it was negative. I took another test at day 38 and got a faint positive. I took one again first thing the following morning and still a faint positive. Two days later I took one more because I had it and it was a darker positive. I am a pediatric ICU nurse and am going crazy with all of the possibilities. My doctor said not to worry yet, we would repeat the US in two weeks. Has anything like this happened to anyone before? Any thoughts? The dates are just so off based off of my LMP. I am 35 years old and this is my first baby, I'm very scared.