I had sex about four weeks ago and partner slipped a little so I took plan B 2 days later and pregnancy test was negative at the time I had sex again 10 days ago and pregnancy test is positive as of 3 days ago. When did I get pregnant?

There's no way to know for sure when you conceived. If there's a question of paternity you'll have to wait until the baby is born and have a DNA test done.

Conception dates are guesses, not exact, especially since you had sex twice, and there's no way to know if the plan B worked. It's not 100%.

But judging by the fact that you only had sex 10 days ago, it's unlikely that you're pregnant from that encounter. I would guess that you got pregnant with the first encounter.