My period i had it last week & i'm bleeding again what can this be ?

Breakthrough bleed: It's common to have some breakthrough bleeding and it's typically due to the fact that you have some hormonal imbalance. Many women experience this. Especially at 16 years of age, it's common for there to be fluctuations in hormone levels. If it bothers you, can consider birth control pills --- hormones---- to help normalize the cycle and prevent breakthrough bleeding.

Period: Periods vary widely from woman to woman. Some periods are punctual, some are unpredictable. See your doctor who may prescribe you oral contraceptives to regulate your menstrual period.