Lower back pain in a 13 year old?

I'm a 5 foot, 101 pound, 13-year-old female. A couple months ago I had really bad pain in my lower back (almost to the ocks area). It was gone within a night, and didn't come back until one night a few days ago - but it was gone by morning, too. But it came back sooner - just a day later, and started in the morning, but was pretty much gone by 1 o' clock. It hurts more if I lean back or forwards at my waist...um, WHAT could it be? (And is it genetic, possibly, because my 40 yr old dad has back pain sometimes...?)