It is not uncommon for an ultra sound done at 8 weeks to be rather disappointing; This is why routine ultrasounds are usually done at/after the 12 week mark (unless there is a problem that an early ultrasound may detect.)
I think it is most likely your dates are wrong ... and you are not as far pregnant as you thought. I also think it could be the fact you have a 'retroverted' (or tilted) uterous which makes it difficult for a sonographer to detect a fetus abdominally in early pregnancy - in which case a transvaginal ultrasound would probably alieviate your fears.
In any event - if you still have all your pregnancy symptoms then chances are good that you'll see your healthy baby via ultrasound in the coming weeks. It is unlikely that you have miscarried and not noticed; you haven't mentioned any bleeding/cramping concerns.
Worst case scenario would be that you have a 'blighted ovam' (pregnancy without a fetus) but that's a long shot if you were to ask me!!! I think it's probably one of the above mentioned factors and that worrying this early, while understandable, is just going to do more harm than good. Take good care of yourself - and bedrest wouldn't hurt if you felt really apt to give your bub the best chance of suceesfully embedding into the lining of the uterous wall. Other than that - it's really beyond your control.
Hang in there! I had 3 miscarriages after 2.5 year old DS was born (one of them at 13 weeks) and I am now 36 weeks along with our second, due 14/7. I spent too much time an energy worrying about miscarriage during the first trimester that I didn't stop to smell the roses and enjoy that time. The best thing I did was realise that it was out of my hands; All I could do was eat well and look after myself. From that moment of realisation on I have been having a wonderful pregnancy!!!