What can cause you to miss a period?

I miss periods alot though I also have ual every 4 months protected of course with condom and pull out method.But I would like to know things that can cause you to miss periods.It happened to me before I skipped like 3 but then got mine again and that happened when I wasn't ually active I am 21 so im not in menopause,so what could make you skip periods.