Can you get pregnant on the last day of your period?

Sorry if the title was a bit much..

I recently discovered that I’m pregnant, but I’m not sure if it is my exes baby or the new guy in my life.

The last day of my last cycle was the day after Thanksgiving. We had sex that day.
Things happened and we split the next day.

I started talking to one of my friends, and we hooked up about a week later.
Don’t judge me.. I don’t want to hear it.. I was on Birth Control, but we didn’t use a condom.
We have had sex twice. Once a week later and then 3 days after that.

So my question is… is it possible that I got pregnant with my ex?
Or is it most likely that it’s the new guy?

I’m stressing out.. so please don’t be harsh..