High hcg

Did anyone else have abnormally high hcg levels with just 1 baby? I am scared. I am 8 weeks 3 days pregnant with 1 baby and just got my blood results back from 5 weeks and my levels were 17,000. My ob thought I may be further along but an u/s just showed I am 8 weeks 3 days with 1 baby. Why are my hcg levels so high!? I'm freaking out. Has anyone else had this happen? What was the outcome?