7 week sonogram, no baby visible

I was wondering if anyone had any advice or similar stories -- I recently went in for my first sonogram at 7 weeks and the technician informed me that she could not see the baby and that the gestational sac only measures 5 weeks.  My progesterone was slightly low when I went in the week prior, but nothing too concerning.  I think it was 13.6 and my doctor put me on supplements to help boost that number.  My HCG levels did not double over 48 hours as I hoped, but they did double over 72 hours.  My doctor said that the news about the sac looking empty is "not great" but she did give me some hope by telling me that they've been fooled before and that sometimes the baby can be hiding.  She also mentioned that I could have ovulated late.  I was using a fertility monitor at the time and it told me that I ovulated around day 18 of my cycle.  That might explain the gestational sac measuring a few days smaller, but I am not sure whether that could justify it measuring only 5 weeks.