There can be a lot of factors going on here and like Knott, I could blather on for hours but I am at work and have a patient coming in right away.
I agree with the idea of a muscle imbalance and core weakness. Hip stretches can help a lot and working on cycling specific core strengthening exercises are great.
Another way to take out a bit of the reach in your fit is to go to a narrower handlebar. Most women are fit best on a 38 or 40cm handlebar but there are ways to measure what would work best for you.
Knott will probably get back to this before I do, but I'll try to post some stretches and exercises you might want to try later.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS