How early can I test positive?

I ovulated 24-25th of July and got a positive August 5th.

Last Monday I had my first ultrasound measuring at 6 weeks and the sac at 6W6D.

Does that sound right? Or should I have been measuring at 9 weeks?

Today I should be 6weeks and 6 days if the baby is measuring correctly. From the ultrasound.

My dr wants to wait till October to even do anymore testing.. I’m worried something else is wrong.