No visible GS

i went to the Gynaec yesterday at 4+4 weeks as per Lmp which should be quite accurate though as i had tracked ovulation. Doctor says he is unable to locate the sac through transvaginal scan. i asked him if its an ectopic pregnancy but he says cant say anything now as it could be too small or bcoz your too obese and asked to revisit after 6 days.

This is my second pregnancy and during the 1st it was visible at initial stages itself. I'm worried.

Is this normal or happened with anyone and has successfully pregnancy.