Just how much does a girls time of the month effect how she acts?

I get really moody when its my time of the month. i sometimes cry for no reason and i get mad easily. it could be that if shes insisting nothings wrong, but usually "nothings wrong" means something is wrong. just kind of stay out of her way for the next few days and see if things change. maybe its a problem that her girl friends could help her with better. dont take it personal. it might not even have nothing to do with u. just act like nothings different