ultrasound today might be having a M/c

Went in today for early scan due to light scant brown discharge, they couldn't find a baby only a small sac doctor said looked like 4-5 weeks along but based on my period I should be 7weeks 2days. They decided to draw my blood, blood came back around 2100 which doctor said was early number close to 5weeks. They want me to come back for a scan Thursday and more labs. Has this happened to anyone else?