Ultrasound scare

So today was my first ultrasound. My husband & i were so excited to see our 9wk baby & to hear it's heartbeat.. however when the ultrasound tech hooked me up to the ultrasound she could find a baby...so she did a vaginal ultrasound, which revealed a sack & egg but still no baby. She said i was measuring at 6wks which is odd considering my last period was December 3rd. I go back in 2 weeks for a follow up basically to see if the sack has grown or if my body has rejected the fetus....

Has this happened to anyone before? I mean is it common to measure less weeks then expected?

I would appreciate all inputs.