Not so good news after first ultrasound

I had my first ultrasound today, the baby should have measured around 8 weeks, instead it measured 5 and a half. My last period was 11/19 and the last date we had sex was 11/29. I doubt that I miraculously conceived late, my gut says we're going to loose this baby. :-( They took blood today and I go back Monday for another blood test, looking to see if my hormones increase. Tuesday another ultrasound. Has anyone else experienced something similar?