Can’t stop overthinking, my girlfriend is pregnant

My girlfriend is pregnant but my overthinking is killing me for some reason my brain is telling me it isn’t mine and I don’t know why.. we’ve been trying for months, she stays here 3 days a week and the 4 she doesn’t we speak all day every day, we even have eachothers location

We found out when she was ovulating and she stayed mine for the 3 days before she did but my brain just can’t shift it

She gives me no red flags or anything and she’s such a perfect girl.. how do I stop these thoughts? They’re giving me major anxiety

I was cheated on in the past maybe that’s why?

I’m 26 she’s 25