Periods are measured in days from the first day of bleeding to the until the first day you start bleeding again.
You won't get your period on the 15th of every month because then each period would have to know to change.
They will be somewhere between 24 and 32 days between the first day of each approximately.
It can also take a few years for your periods to settle into a pattern.
Avoid situtaions that may lead to getting come in/around your vagina.