Not necessarily! Irregular periods are very common; in fact a third of all girls have to deal with irregular periods. If you just started getting periods, don’t worry if they're sometimes late, as you get older they start to arrive like clockwork. There are other causes of irregular periods including hormones, stress, STI’s (sexually transmitted infections) and pregnancy. If you think STI’s or pregnancy could be the cause of your irregular period, head to the doctor as soon as you can.