Just wondering if anyone has had an ultrasound and been told they weren't having twins only to find that they were indeed carrying two babies?

I have had extreme nausea and very high HCG levels all the way (am 12 weeks now) and have a huge bump already (measuring the same around my waist now as I did when I was 5-6 months with the other pregnancies). I had an ultrasound at around 7wks and only one tiny sac was seen - but I'm not convinced.

Have I just been day dreaming too much? How often do you think they are incorrect at 7weeks?