Ovulation timing

So I had the Mirena so I didn’t really have a consistent cycle. I got off my Mirena in June and after it was removed I had 30 day cycles. Well I just found out I’m pregnant and I am trying to find out when I conceived. My positive was on the 16th and it was very faint but I got a blood rest done the next day and my number was in the 60s. I had “sex” (he didn’t go inside me at all he came on my stomach and a little on my underwear but I wiped it off with a towel) 2 days after my period ended. I couldn’t get pregnant from that right? Thanks for all your insight!