Sac but no baby.

I went to my first baby appointment today and they did a transvaginal ultrasound and they saw a sac but no baby. I'm so heartbroken. According to my LMP I'm 7 weeks 4 days. The doctor said that maybe I miss calculated. I found out I was pregnant on the 16th of last month and it was a really faint line. They took blood and I go again Thursday for more blood work to see if my HCG levels are going up,if not that means I'm going to miscarry. Had anyone had this happen to them and ended with good news?