Freaking out right now!!

So just saw the doctor today, and my serum hcg was 450!!! I'm only about 4 weeks, 16dpo. I got a positive clear blue digital test at 10dpo. Now I have to have an ultrasound next week to find out why my levels are so high so early. Has anyone ever had this before? I'm freaking out that this pregnancy might not be viable after trying for over 4 years! Please help me!