The least pregnant...? Dates (weeks) help please

Ive had some weird cycles lately and I have had an empty scan today. I'm just wondering if maybe it could be too early but in my very emotional state I just cannot figure out what the least pregnant (weeks) I could be. Please could some one help? I had my last period 13 September. My first positive pregnancy test was 17 October with a very faint line. Doctors confirmed on 26 October. Scan today showed sac about five weeks but no baby. By my lmp I should be seven weeks ish. Is there a chance I could be less than this and that's why there was nothing on my scan? Thanks in advance ladies x