7 week ultrasound shows yolk sack but no baby

Hi everyone. I am supposed to be 7w+2d but on Tuesday I went for my first ultrasound (6w6d) and the doctor only saw a yolk sack with no baby. They informed me I was most likely miscarrying. They did an hcg test on me and called me today with results.. It came back at 61,000 and nurse said with that level something should have shown on the ultrasound. She started prepping me for what to expect during a miscarriage etc.. I'm just so sad. Has this ever happened to anyone and then the next us there was a baby there? I am going again on Tuesday and getting another ultrasound to confirm ..