Er Doctor says possible empty sac high hcg levels

Found out Tuesday November 17th that I was pregnant after having a miscarriage in June of this year.  Lmp was 10/22. Went in Wednesday and had blood drawn levels came back 261 went back for follow up on Friday and levels were at 871. I just left the Er due to period like cramps and had blood and ultrasound done. The sack was visible on ultrasound and it looked like there was something in the sac but the tech said if it isn't clearly showing then the doctor will say there's nothing there yet. The doctor came in and said that clearly I am pregnant but I am having an abnormal pregnancy. He said according to my ultrasound I should be 5 weeks and 4 days but my hcg level shows that I should be farther along then that. He said that with level of 8690 that he should have been able to see heart rate and fetal pole and there wasn't one. When he told me this I broke down. I am so excited and love this little one so much already and I'm scared that I will loose this one too. Has anyone experienced something similar to this? Y