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!

Ithink the answer is 1. warmth

Keep your breathing calm and try to ignore it