Confused :(

I had my first ultrasound today. The doctor and I estimated myself about 7 weeks 1 day. We proceeded to go on with the ultrasound. We didn’t see what we were expecting to, but we did see a amniotic sac and a speck where the baby would be. He said that he’s had many cases such as mine and he has had them come back a week to two weeks later and there was a baby floating around. He believes that when I have my next ultrasound in two weeks that we will see a little baby. I’m just extremely confused. My doctor believes a ovulated late. My last period was December 17th and was 4 days long. I tested positive on January 13th with a dark bold test. Implantation takes 7-14 days so I can’t possibly be only 5 weeks can I? I was figuring around 6 weeks maybe. I’ll know at my next appointment it’s just been a very confusing day. :(