How To Calculate The Conception Date?

I'm 19 years old 5'4 and 130lbs. Okay so according to the doctors I'm 5 weeks pregnant. And I was told they go by your last lmp which for me was may 8th and I was due june 5th. Soo realistically am I really only 3 weeks because I only had sex after my period. I had sex may 19th and everyday after. I was ovulating around may 22nd too. So did I get pregnant from around that time? Because this might be an issue with my bf cz we didn't have sex for the first time (with each other) until the 19th