I am A little confused?

I have been reading a lot of books and websites, and I know it is not fair to compare pregnancies with a stranger because every woman is different. I went into the ER on monday for some lower abdominal pain and I had a u/s and levels checked. According to my Lmp I should be 7 weeks, according to u/s I was 5 weeks and 2 days. I had no bleeding or spotting, but I had a subchorionic hemorrhage with my last pregnancy that caused a lot of cramping so I just wanted to make sure it was not the same deal this time. My blood was drawn and my levels were checked, ultrasound showed a gest. sac with yolk sack (too early to see I know) that measured 5 weeks and 2 days but no fetal pole. I asked the nurse for my hCG results and she told me 18,075......this makes no sense to me because that number does not at all correlate with a pregnancy age of 5 weeks, unless there are multiples which the u/s tech did not see......What in the hell is really going on???????? I already called the doctor and asked for my results again because maybe she read the number wrong but this is frustrating and confusing as all hell........**sigh**