Only 5 weeks along?

The first day of my last period was 8/11 but my cycles are always irregular. I took a test on 10/11 and it was negative, took another on 10/12 and it was VERY faint but positive. I went to the dr and my hcg was 73 on 10/13, went back on 10/19 and it was 1465. I got a little excited but was still nervous about another mc. We went to get an u/s yesterday and all she saw was the sac and the yolk, no baby yet... So she guessed I was 5w3d. Now I'm worried, when my hcg was 1465 I was only 4w? My dr wasn't very helpful, she just kept saying it must be real early and told me to come back for another u/s in 3 weeks.