Measuring large....

I went in for an ultrasound today just to comfirm pregnancy so I figured I was about 4 weeks ( we used opks) the doctor said they guess I'm around 3weeks 6days but then the ultrasound showed a large sack at 5weeks 2days so measured a whole week head which we no is impossible we i was on my period at that time plus the opks were spot on with the app for the most part and i got my bfp at 11dpo because at 9dpo vvvvfl....my question is could this mean identical twins because my DH and my family have twins on both sides i know there isnt evidence to support men carrying twins but my grandfather carried the gene from my dads side and it was identical and my grandmother on my moms side had also twins( unknown if identical or not)