empty sac at 5 weeks!!!

Last week I went to the doctors because I wasn't feeling well and nurse told me I'm 7 weeks pregnant. I recently went to my first ultrasound and there was nothing but an empty sac. The nurse measured my sac and told me that I'm 5 weeks instead of 7. I have to go back again in 2 weeks to see if there's any baby growth. I'm losing my mind!!! Has this ever happened to anyone?!? I was so excited to hear my baby's heartbeat and now I'm being told I have to go wait 2 weeks to see if there's any development. Any advice???