Could the ultrasound have missed a baby?

So I recently had a miscarriage and ended up pregnant again within a month. I got sent for a dating scan which showed me at almost 6weeks. However this pregnancy my symptoms have been stronger. I started showing at 7weeks which I thought was just bloating but it’s still there and showing more now my mother even made a comment when I was visiting her (10weeks today). My dr doesn’t want me to have another ultrasound as I had then constantly last time only to lose my baby so if I want one I’ll have to pay for it as it’s not covered by Medicare. Is it possibly the first scan didn’t show a second as it was so early?