What causes PMS in a girl?

Hormone imbalance. Most girls with pms have a relative Progesterone deficiency in the second half of the cycle. Vitamin deficiencies are also thought to be causative. I get the best response with Progesterone the last two weeks prior to menstrual bleeding. Generic Prozac (fluoxetine) works well the week prior to menstrual bleeding as well.
Hormones & evolution. Pms may result from alterations in or interactions between the levels of sex hormones on brain chemistry. It signals failure to become pregnant. The mood swings may be evolutionary - a woman has only a fixed number of cycles to become pregnant before reaching infertility at menopause. Pms suggests reassessment. Do i need a new mate? Am i eating right? Am i ok? Fair or not, it ensures survival.