ultrasound question

Okay so I'm suppose to be 7 weeks 6 days , I went to an ultrasound around 5 weeks and their was NOTHING , No sac or anything. Today I went back and their was an empty sac. May I have just ovulated very late or is it most likely a miscarriage? last week my pregnancy tests were getting darker and darker everyday. So I'm hoping I just ovulated very late...