HCG level help

So I had some bleeding Sunday and Monday. I had my blood drawn Monday, which I would have been 5 weeks 4 days. I just got my results today, and she said my levels are high, 900. So they are having me get an ultrasound Friday. I am totally ok with that, but I am very confused. I thought 900 was a normal number for 5 weeks. What are your thoughts or experiences??? Thank you ahead of time.