Teacher + period + ask why we have to go to the nurse??!?

Heyy! I am going into eighth grade. I have had my period for the past two years and I have super super bad cramps. My friends in 8th grade said the teachers always ask why we go to the nurse. I am super super modest so I don't wanna say I have my period. Its really embarrassing! (something like that happened when the teacher found out I was on my period last year) so lf I need to go to the nurse and the teacher asks why, what should I say? PS Im bad at Lying hah oh and the teacher is female