Having 2nd period after only 12 days from last cycle what could be the cause?

Early menses: Have you had irregular menses in past? Are you on any hormonal birth control? Both of these could contribute to having a period earlier. Some women can get early bleeding right after ovulation if there is failure of pregnancy. Even stress can cause it. Usually, it is not dangerous, but if it continues, you need to see a doctor for an exam. Hope it helps.