Many women suffer of this, I don't think it's related to posture. Gym must be of help, I'm sure, but still.
All modern women have bad posture. Busty women suffer more from having bad posture. There's also just the Spaghetti-Strap Tyranny of clothes being designed for women with flatter chests, so women feel compelled to operate on their bodies to fit into bras with thin straps. It's a modern sort of foot-binding.