HCG too high?!? 4 weeks and 6 days.

What is a reason hcg would be too high? My last period was on June 14th. On 9 dpo I got my first faint line. 10 dpo it was darker and I registered on a digital. At 11 dpo my hcg was 32. At 13 dpo it was 98. They were very happy with these numbers. However, I went in yesterday at 18 dpo and they didnt tell me the levels but they want me in thursday for an ultrasound because they are high. I am trying not to stress out. I will be 5 weeks tomorrow. I had a miscarriage in October last year. And a tubal in March of this year. With my tubal I constantly spotted and had very low hcg. What are reasons it would be too high?

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**UPDATE: My numbers were 888 at 18 dpo. I called back and asked. I do not thing these are high. I think they are great for how far along I am. I am 5 weeks tomorrow. But at least I get an ultrasound to be sure everything's alright.