Usually period means no baby correct?

Why all of a sudden am I hearing these "I didn't know I was pregnant" and "I had my period the whole time"

Did you have this happen?

It must of differed in some sense from a normal period right? Shorter? Lighter? Or was it really like Aunt Flo being high and mighty? Lol.

I'm not trying to put anyone down I am honestly curious and I am not going to leave it to Google.