The topic of menstruation can be private or sensitive in many cultures, so use the word respectfully. She took a day off for health reasons; she's menstruating.
Pro Tip 2/3
Gender Awareness
Be aware that not only women menstruate; transgender men and non-binary people may also menstruate. Alex is taking testosterone but still menstruates occasionally.
Pro Tip 3/3
Biological Process
Menstruation is a natural biological process, not an illness or problem. My biology class taught me how menstruation is a part of the reproductive cycle.