Please help!

So ladies i am very concerned and confused, on the 18th of december i had a D&C for a missed miscarriage i was 5 weeks. Then i started taking pregnancy tests they were negative a week later. On the 9th of Jan i got a positive OPK, and on the 20th of jan i got a faint positive pregnancy test. Fast forward to now, this week i had two ultrasounds, one says im 8w3d and one two days ago says im 9w. And according to glow i should be 8 weeks 5 days. Whats worrying me is that my dr told me she believes this is the same pregnancy of December and i was probably carrying twins. And they removed one and one stayed. But wouldnt i be with more than 9 weeks if it was that pregnancy? Seeing that on dec 18th i was 5 weeks already? Wouldnt my pregnancy tests remain positive throughout Jan? Please help im sooo confused and concerned with this situation i dont know what to think. I always counted as a new pregnancy from my new positive ovulation test