1st Ultrasound today at 5 weeks and 2days

I am very nervous because my ultrasound showed a gestational sac of 4 weeks 3 days but I should be at least 5 weeks and 2 days since my last LMP. I am still waiting on my HCG levels to come in but should I be worried?? The tech lady said maybe I ovulated later but I used a clearblue fertility kit so I know when I ovulated! If can be that far off!