6 weeks 5 days but no gestational sac

Hi All, just went for my first vaginal scan and doctor could not locate any gestational sac. He can't explain what happen and ask me to stop medication and wait for menses to come. My HCG increases slowly from 526 (14 days ago) to 896. Anyone experience this and able to share more with me? :(#firsttimemom #pleasehelp #advicepls #firstbaby