Period Started on First Day Of School

I went to my first day of my junior year today feeling good until around 2nd hour when i started to feel like maybe i had started my period but i didn't think i was, because i had just checked this morning and i had 8 days left. I went to the bathroom after 4th hour and noticed i started and was so annoyed. The rest of my day consisted of very agonizing cramps and worrying if i'll bleed through my light pants. luckily i had packed tampons this morning and i made it through the day okay. when i got home i changed into sweatpants and layed in my bed and i feel much better👌🏼