Concerned/Worried ultrasound results

My original due date given to me by my doctor based off of my LMP (June 13)  was March 20,2015 making me 6wks5days . I went in today for my first ultrasound and I was told they were unable to detect a heart beat and that I was measuring only 5weeks5days which too early to see one . Instantly I was concerned and they said they want me to come back in another 7-10days to have another ultra sound .. What concerns me is that means I was only 2 weeks 5days when I took the at home pregnacy test is that even possible ? And if it isn't then that means the baby isn't growing ? I haven't had any bleeding or cramping so the doctors say just wait it out another week .. I'm so worried !! Has this happened to anyone else ?!