Excuse letter due of menstruation cause of stomach ache?

Dear [Recipient],

I am writing to inform you that I am unable to attend [event/meeting/class] today due to severe stomach ache caused by menstruation. I have been experiencing intense cramps and discomfort, and I believe it is best for me to rest and take care of myself at home.

I apologize for any inconvenience this may cause and I assure you that I will make up for any missed work or responsibilities as soon as I am feeling better. Thank you for your understanding and consideration.

Sincerely,
[Your Name]