My 15 year old cousin has had her period for three years now, but for the past 2 months its been happening twice in one month, is this normal?

Frequent menses: At her young and tender age it can be considered normal. She may be exercising or training too strenuously or being stressed or may have been ill. Many other factors can come into play as well.

This may happen.: Make sure to have a menstrual record chart and mark the vag. flow daily as it happens. Wait another few weeks and if the irregularity seemingly continues, see a gyn.