Dear fellow my periods statrt after just 15 days may I know the reason why does it happens... I'm realyy worried about it please help me out... THANKING YOU..?

This is not necessarilly a sign of anything wrong. Just because it is uncommon does not mean it is unhealthy.

How long does you period last?
Have you ever taken any hormone drugs? (growth pills, untested birth control, breast enlargement/reduction, post pregnancy vitimans, etc.)
How long has this been happening? Did it start after anything, or is this how your period has always been?
Have you ever been to a gynocologist?