Why am i getting my period 2 a month in less then 20 days ( i'm 16 )?

Hormones: Most commonly in someone your age it's due to imbalance of estrogen and Progesterone and still developing hypothalamic/pituitary communication with ovaries. Second possibility is weight (thin/heavy). Thyroid can also play a role. And lastly vonwillebrand is another possibility (rare, but not impossible). I would keep a menstrual calendar and speak with clinician about your symptoms.