Having a regular period but somehow still pregnant????

I know, this is wild and stupid and very “someone doesn’t know basic biology” but i’ve been reading up on this since for some reason this topic has been EVERYWHERE! How does someone have a regular monthly period and still be pregnant when literally the whole reason for having a period (at least what i was taught) was because you ARENT pregnant???? Also, to top this off, the articles i’ve read about had women not even showing the entire pregnancy leaving them to believe they’re not pregnant (i mean obviously if you’re getting your period regularly and barely gain weight right???) until they start having contractions. It’s kind of freaking me out since the last time i’ve had sex was back in April and been having a regular period but a bit of weight gain since. LOL really stupid of me to freak out but what if i’m one of the rare cases???? highly doubt i am BUT with all these stories popping up out of nowhere, it’s kinda making me think that maybe this isn’t a coincidence (((i’m sure it is though but you know)))