Given the data you provided, you might be experiencing menorrhagia. It is not uncommon at your age. The period after the menarche (the onset of menstruation) is sometimes accompanied with prolonged menstruation. It is probably due to the hormonal imbalance. However, a menstruation which lasts for 26 days as you mentioned, might cause certain blood disorders due to the loss of blood. Anemia can be treated by an increased intake of iron, but a visit to a doctor is also recommended to determine if hormonal treatment should be performed.