Today was swimming day in my pe class. i did not dress in my swimsuit cause i have my period. my teacher ask me to write him a note. do i just write "because i am on my period? "

No. you should have just worn a tampon. Tampons are water resistant and they dont hurt or get out. They pretty much absorb all the blood.

Being a mom and teacher, I would go to your nurse, and ask them to write something in your behalf.  I'd also inform your parents/guardian and school counselor, because I think it's inappropriate for the teacher to make YOU write a note.  If he insists, just write
Dear Teacher,
I am currently unable to participate in swimming, as I'm having my monthly cycle.
You'd think he'd want more verification, maybe from an adult!
Sigh...good luck!

Ofator que afeta o bem estar e o fator mental