Girls, I'm completely confused — I guess pregnancy is affecting me lik…

Girls, I'm completely confused — I guess pregnancy is affecting me like this ? The doctor keeps asking for the first day of my period. Of course I gave the date when it was supposed to start, but it didn't. And last time the doctor specifically asked when my last period was — from May 4 to 10. Only now did I realize that I should be giving May 4 as the date! And now my gestational age is almost 16 weeks. At the check-up the doctor says 11 weeks, and I haven't had an ultrasound yet. ? Please tell me how to calculate the date correctly, I feel like Dory the fish ?