Why are some women irritated during periods?

Anonymous
Yesterday I ran into a friend at Walmart
He said his girl friend who moved in with him recently was having periods so he was there to buy some extra tampons as she requested.
He also asked is there something he can do?
I said some chocolates, flowers and kisses works in my case and he decided to follow the same
However today when I met him , he said she was rather angry at him and yelled at him for no reason and said things like just be thankful that you don't bleed like this and everything is easy for you guys etc etc
It was shocking for him since he never saw her like this ever before and said it's probably due to the mood swings and she didn't mean that.. is it?
(She did apologize later today)
Why are some women irritated during periods?
Why are some women irritated during periods?
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