When did I conceive 🥴

Hello, so I found out I was pregnant a couple months ago & currently trying to figure out when I conceived. My first ultrasound was July 17th and was measuring at 6 weeks and 5 days. The last time I had sex prior to that was June 11th-14th with the guy I’m actually with now but before I got with this guy I did sleep with an ex May 24th. My periods are irregular so it’s hard to go off of that. So on July 17th I don’t see how I measured 6 weeks & 5 days when really I had sex 4 weeks prior to that date, June 11th-14th. Somebody please explain to me how and when I conceived so I can make sure I’m making the right decision.