How To Determine The Exact Date Of Conception?

I have a fertility question. My last period was on the 12th of september, I was late as well. I usually come on around the 9th. I broke up with my guy around this time. On the 21st of september I had protected sex with an old fling. Now before I had sex with the fling I was already thinking I was pregnant and took a pregnancy test. The next month my period was a no show, and I took a pregnancy test and I was pregnant. Per ultrasound I was 5 weeks pregnant with a due date of June 19th. Now to be sure I calculated and I was a whole week ahead of when I had the protected one night stand, it would of been impossible for me to be pregnant by him. So my question would be am I right about excluding the one night stand as the father?