anything and I mean ANYTHING can mess up your cycle. sleep pattern changes, diet changes, changes in exercise, mood, anything. when I went off birth control for almost a year to get my PCOS in check I was regular at first. Then I got an internship and started doing physical labor in the forest 40 hours a week. I didn't get a period for 9 weeks! Then, the week after I came back from my internship and my schedule went back to normal I had a month long period. Then no period again for 6 weeks.

If youre not on birth control that keeps your hormones stable, anything can cause your period to delay or go out of whack in any way. it really doesn't take much to do that.