My 1st ultrasound says that I'm 5w and 2 days pregnant. The date of my ultrasound was October 2. So when does that means I had sex with my boyfriend? What date? Because I can't remember my last menths period (LMP) help!

Somewhere around Aug 10 to Sept 8

The testing is good at best with give or take a week either way.
But can be off at times up to two weeks give or take.

There is never a way to know any exact date.

If you can't remember the days you had sex, we will not know. Conception dates or even due dates are just "best guess" never exact

I do remember I had sex with one guy I guess around aug 27 I'm not sure. But it was kind of like a one night stand. Then my boyfriend and I had sex aug 10 &11.