My second ultrasound showed an empty embryonic sac. How is this possible?

Hi doctor,

I am a mother of two kids. My last period was before 69 days. I am worried and I took a urine pregnancy test before 20 days. It showed me a positive result. I have my usual pregnancy symptoms. Last week, I had some spotting in chocolate color. I took my ultrasound before five days. The doctor told me that the baby is fine. But still, my spotting has not stopped and so I decided to consult another doctor. I took my second ultrasound before two days and I was able to see only an empty sac. How is this possible? The doctor suggested me to come after two weeks for taking another scan.