Had an ultrasound today, was supposed to be 6w6d but the scan showed a sac that has a gestational age of 5weeks 2days. Could it be Blighted ovum?

Maybe. It could very well be that your dates are off as well. I would repeat the ultrasound in a 10 days or so. In addition, it would be of value to check to hcg levels 48 hours apart to see if rising or not. If dropping then that is a sign of a non viable pregnancy.