I agree with @blakeyrakey , it can take some time before your body adjusts to having a period every month. The period that you are describing is called an irregular period. It’s TOTALLY normal, so don’t worry! This usually occurs close to when you start your period, but it can (it doesn’t usually) occur at any time.
Also, some good period tracking apps are Clue, Flo, and I think Eve is pretty good.
It takes a while for your period to be on a regular cycle/ also! Some ppl have cycles that are 21 days and some are like 34. Everyone is different. Track it the next couple months (make a note in your phone when you get and when it ends) so that way u can start to see your cycle. There are also some good period apps but I don’t know the names. Anyone else know them?