Ovulation question?

Based on my last period, I should be 9 weeks pregnant. But my HCG levels are around 6 weeks and an abdominal U/S didn’t show the baby.

I took a positive test on March 27th, an early detection.

So how would it be possible for me to be around 6 weeks pregnant if I found out 6 weeks ago with a positive test?? I’m so scared and confused.