A lil scared

I went to the doctor yesterday excited to c the baby im supposed to be 8w5d when they did an U/S she told me i only looked to be about 5w5d has this happened to any one? From my last period i should be 8w6d today but in the U/S there was only a sac could c the baby or hear a heart beat :( im a lil scared. I go back in 10 day to have another one to c if there a heart beat praying more then anything my baby is okay we been trying for 2 1/2 years and finally got pregnant i think thats why im scared the most.. Please let me no if any one going thru this...