My due date doesn't match w/ the time I conceived

Ok I keep up with my periods on an app on my phone & every time I have sex as well. My boyfriend of almost two years thought I was pregnant so he got me a pregnancy test after my period was 10 days late. Turns out I was pregnant. A couple days after I decided I wanted to go to the hospital to make sure the baby was ok, they didn't do an ultrasound because they said it wasn't an emergency so she got a calendar and asked when was my last regular period. My last regular period was May 5-8, but I did experience bleeding for almost two weeks at the end of May (May 30-June 4) I believe that was my implantation bleeding. But anyway with her calendar she said I should be about 9 weeks. Two weeks later I had my first ultrasound and they told me I was 7 weeks and 5 days, from how the baby measured. The problem is my boyfriend left for 5 weeks for the military May 17 (around tht time) and he came back June 15th. Before he left we had unprotected sex and when he came back we had unprotected sex of course. But he's kind of questioning it because he says the dates don't match and I haven't been with anyone else. He still supports me and nothing's really changed with us but he does have his doubts. I'm also confused as well on how due dates are calculated. But both dates given aren't matching up. As of today my I am 8 weeks and 1 day, but he's only been back for 7 weeks. There is a 7 day difference from when he came back and had sex. I just need to know if I'm the only one here ? 🤷🏾‍♀️😕 oh and my due date is March 18th