It’s probably because everything else is fine! Seriously - your scans are healthy, your pregnancy is going well, so your brain has invented something to worry about. If it wasn’t this, it’d be something else.
And it is no different, you’re right. This anxiety is a sign you’re a good person who takes fidelity and honesty seriously. That’s why your brain is using it against you - but it’s also how I know it’s something you just wouldn’t have done. It’s not in your nature.
It helps me to not give my worries any ‘credit’. It’s a tip I got from an OCD forum. By asking people, messaging people etc you’re acting like this is even a possibility, which it’s NOT. But as you’ve found, no matter what answers you get, your brain will pick holes in them, find a way for them to be wrong.
When I was younger my OCD manifested in health anxiety, and I became convinced I had HIV (for no reason, but as you know that’s how it works!). So I got an HIV test, hoping it’d put my mind at ease. And when it was (of course) negative, did it put my mind at ease? NOPE!!! I became convinced it was wrong, that I was still in the infection window where the disease can’t be picked up, or they’d tested the wrong sample.... because my anxiety wasn’t based on fact, it wasn’t rational or practical, and so rational, practical answers couldn’t cure it.
That’s why I’m not getting a paternity test, or a harmony test, or anything else that’s crossed my mind in my dark moments - because I know I’d be treating the symptoms, not the cause.