Hi Luna,
You raise a great question.

It is totally normal for your periods to actually be quite irregular the first couple years of menstruating while your hormones find a balance.

But once your hormones become regular it is important to recognize that irregularities aren't normal and that they may be an indication of something more serious. There are several reasons your period can be irregular, some of these can be found at https://www.bepreparedperiod.com/blog/why-is-my-period-irregular/.

Hope this helps!