Thanks for query,
You have hormonal
Our pituarty glans
in brain produces precursors of hormons gets stimulated our adrenal gland to produce sex hormones at the age of puberty
In the initial years of minarache girl may get irregular flow of hormones leads to irregular cycle.
So nothing to worry.It may be regular by it's own but if you have prolonged bleeding cycles then
Consult a gynecologist she will give you hormonal medicines for 3 cycles that will learn your glands