Is it possible to be sure of conception date?

Nunu Mother Of 2

Okay, I'm 36 weeks and this has been honestly worrying me the whole pregnancy. I'm confident of who my baby's father is but I haven't told anyone so here's me getting it off my chest:

My last period was 8/10/18 and my Flo app said I would be ovulating on my birthday (8/19/18) and on my birthday I had a jacuzzi suite with my partner and we made love like 4 times lol

Around the beginning of September he told me I was pregnant and told me to get a pregnancy test, which ended up being positive.

Now, the confusion is all that year (2018) I had MULTIPLE sex partners. My partner knew of them and we were operating in an open relationship. I properly used protection with everyone except my partner tho and I've never gotten pregnant before while actively using protection.

But my question is this, how likely do you think it is to get pregnant using protection? Please humor me, I feel like I know in my heart its his baby, I just get these nagging thoughts sometimes even knowing I fully and constantly protected myself