Help PLEASE! I’m so confused and stressed

On sept 5th I had an U/S done .. doc measured the actual baby at 5w5d .

Today I should be 6w7d ... began bleeding and went to a different doctor due to moving to a new city. She said the sac is measuring at 5w3d but saw NO baby. Only a yolk sac ...

Has this happened to anyone else???? What could this be ????