Measuring 3 weeks behind

This has been a roller coaster for me the last several weeks. According to my LMP I was 8 weeks 2days, however a few weeks ago I was told I was having a threatened Miscarriage because I has some spotting and my HCG levels were really low <200. I had done three draws and each time they went up but did not double. Well on Sunday, I started bleeding again, so I had an ultrasound today.  The sac was there measuring 5w1d. Doctor said the size of the sac matches the lower HCG levels and thinks I may have just conceived later, but since I am bleeding he sent me for another blood draw to see if my levels have risen since February 19th, and I'm waiting to hear back if the # is above 5-6 thousand. He said if it's not I'm having a miscarriage. I'm so nervous.. 💔