mossed miscarriage?

today I had to call my doctor for some severe cramping and I went to get an ultrasound done. according to my blood work and last period I'm supposed to be 8 weeks. well, they didn't find a heart beat and said I'm only measuring 6 weeks along. they told me that I have to wait a week and see if the baby grows. if not I miscarried. they think maybe I just wasn't as far along as we thought I was. anyone had this happen!? I am terrified!!!