When I was your age, I did have irregular
menstrual cycles and one month just had a show that
after a week progressively got heavier and
after another week of extremely heavy bleeding and extreme and severe pain and joked about giving birth, went to the loo and heard a loud plop. I had been 6 weeks pregnant and didn't know about it.
Not saying that you are, depends if you have been sexually active.
If you have been tired before, it is possible that you may be anaemic.