It's very obvious that you did not conceive on the night out and did not have sex with your boss or the taxi driver. The dates, scans etc prove it and as others have said, you would have had some inkling of what had happened especially as you can remember some aspects of your interactions with them.
But I really don't think that anyone here- or your boyfriend, boss, whoever- telling you this is going to stop you worrying. You will continue obsessing about this until you sort out what is underlying it.
I think you probably know that you are fixating on a completely irrational fear as a symptom of some deeper issues- possibly depression?
I say this gently, with the greatest sympathy/empathy, as I have suffered from depression and anxiety myself, and along with it came the most horrendous fears about myself. It's like my mind conjured up the worst thing I could possibly believe about myself, and tried to convince me it was true. I really believed I was a bad person.
Now that I am well I see how utterly ridiculous these fears were.
It is worth noting that for me, the fears emerged again during my first pregnancy- a time of huge physical and emotional change.
Please talk to your GP again about getting some support.