Pregnant on period. How would you know?

Sigh. So I mentioned to my mom I need to see an obgyn because I get really sick before and during my period. I posted about this here &  had gotten feedback that this was both normal in a sense and also that I should seek help from a doctor. The first thing my mom said is 'I hope you're not pregnant. You can be pregnant and have your period. ' I know that I'm not.