I'm six weeks pregnant and I'm uncertain of my last period, when did I get pregnant.

Usually the scans etc will give you a fair idea as to what date it was

How do you know you are six week pregnant if you don't know when your last period is, since that is often a judge or guideline for the estimate.

But unless there are two men you had sex with close to each other, it does not matter, since estimated dates are just that, guesses. It is give or take a couple weeks at best

I would say 6 weeks ago. Unless your periods are irregular