The last menstrual date was on 21st Oct 07, supposedly right now I'm already 8 weeks pregnant.But the last ultrasound is not showing any sign of a featus. Can the date be wrong ?

Yes, conception dates are guesses based on information you provide and some basic tests. Also there is always a chance your ulrasound tech was not good at the test ( sorry but during my wife's last one she was told all sorts of wrong things, we finally changed doctors)

So follow up with your doctor and see what he recommends