Welcome to HCM,
Pain in the left belly button could be due to many reasons like gastritis
, renal colic
As you got your periods it could be due to premenstrual symptom also.
As you are not using any birth control methods, i advise you to rule out pregnancy
by doing urine pregnancy test.
For pain you can take anti spasmodic like dicyclomine
available over the counter.
For your early periods you can wait for one more month and see if they become regular.
If its irregular or associated with increased bleeding or any other symptoms then consult your gynaec doctor.