2-3 weeks pregnant test

Hi everyone,
I was supposed to have my period on March 14th, but it didn’t happen. I lost some blood for 2 days, which I thought was lighter than usual.
On April 19th, I decided to take a pregnancy test, and it came back positive. On April 20th, I took another test that said I was 2-3 weeks pregnant.
Since I’m only 19 and this was unplanned, I went to the doctor and had an internal ultrasound. The result confirmed that I am indeed about 2.5 weeks pregnant according to the scan, but about 4 weeks according to my cycle.
I have a few questions: When I search online for what happens during which week of pregnancy, should I look at 2.5 weeks or 4 weeks? And how long am I pregnant—2.5 weeks or 4 weeks—if I want to check the due date? Also, am I really pregnant now, or only after 3 or 5 weeks, as I’ve read on many websites?
If you have answers, questions, tips, or stories, I’d love to hear them.
Best,
Iris.