Why is my anxiety giving me irrational fears?

Okay so first a disclaimer: stop reading this if you do not have knowledge about/ have experience with severe anxiety, because it probably won't make sense and I'm not looking for people to act like I'm crazy.

So I've been with my baby's father since November. Around that time my addiction to benzos (prescribed for my anxiety) was coming to an end. I got pregnant in January. I wasn't sexual active with anyone besides him from the day we started dating. However, right before him, I had a few sexual partners when I was taking benzos.

This is where I might lose some of you.

So I'm due September 27th and from my research I figured out I was ovulating January 1st - 6th? Something like that. I have text asking my boyfriend on the 3rd to buy me plan B because he came inside me on the 2nd. So OBVIOUSLY that's when I got pregnant. Right? Ok so my anxiety took over when I found out I was out of town on the 4th-7th and I panicked. I began thinking "Oh my god what if I went over to (an ex that lives in that town)'s house and we had sex and I was fucked up and can't remember it" I CANT SHAKE THIS THOUGHT. (this is my home town and I was back and forth the month of November and I did have relations with my ex) I know it's not true deep down. It's not possible. But It's just such an awful thought, that my anxiety isn't let me shake it. Im making up fake scenarios in my head. I just feel absolutely insane. Someone please help me, I need a peace of mind.