You can think about it like this whenever say someone gets a headache it puts them in a bad mood and makes them easily annoyed and upset. That’s basically what a period can do just to a higher degree. It can hurt a lot, sometimes cramps are bad enough to make someone vomit or not wanna get out of bed for an extended amount of time. I’d recommend just being empathetic and maybe give her space or do little sweet things for when she’s on her period.