For the past 3 - 4 cycles my 15 yr old daughter keeps having 2 periods per month (she started at age 11). She is also having lower back pain. Why?

Hormone imbalance. It is not uncommon for estrogen to get imbalanced after a fw years of having periods and lead to more frequent menses. My #1 recommendation is acupuncture. It will straighten out hormone issues quickly! The back pain if form prostaglandins at the time of her period. Ibuprofen or alleve will help...Or vitex or cramp bark (i am guessing the pain is related to her period?).