No fetal heartbeat

So I went for my 7w transvaginal ultrasound yesterday And they told me to call the doctor today. I did and they told me they couldn't find the fetal heartbeat and only the sac. No fetus The nurse said this could be the beginning of a miscarriage. Hopefully it's just a miscalculation. I go for a repeat US next week to be sure. I have to wait a whole week! I'm sick right now! 😩 
Has this ever happened to anyone or does anyone know anybody that this happened to? I'm trying to stay as positive as I can but my heart is just so heavy right now. Thanks for listening. 😢