Periods getting shorter

Hi. I'm 16 and recently I started to begin to have menstrual periods.
I talked with my mother about it and I got some useful informations about it, so you can say that I'm familiar with the process.
At first, my menstrual bleeding was very light but it turned to normal short after.
The thing that is troubling me is that my periods are getting shorter. In the beginning it occurred every 29-30 days but after a few months it started to show every 25-26 days.
I don’t know what could be the cause of this, nor does my mother.
Any advices?

Well, I'm a woman also and you understand why I do have some knowledge about this.
Irregular periods are very common feminine problem. An irregular period can include a late, early or missed period, or bleeding between cycles. You might also have a much heavier or lighter period that lasts for longer or shorter than usual.
A 'typical' regular period is a monthly menstrual cycle that is 28 days long (four weeks), with period bleeding lasting for 4–5 days without bleeding in between.
These abnormalities are in most cases a normal thing and they are caused by hormones peaks. This could be provoked with some emotional stress, changes in feeding habits and taking some oral contraceptives.
If you haven't had sex recently and aren't taking hormonal contraceptives, your periods will probably re-set them selves or settle down.
If this doesn’t happen contact your gynecologist.