Pregnant and don't know who the father is!

So I know it's bad but I had unprotected sex (casual relationship) on the Friday then I ovulated on the Saturday and slept with my ex (I was intoxicated) on the Sunday 😔 
Now I'm pregnant and I have NO idea who the father could be... And I'm not going to point fingers at either one unless I was 100%
Does anyone have a better understanding of the science behind this and could offer an educated guess as to who might be the dad? 
TIA x 

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