because of breast sagging back does not ache. there may be some other reason.
for breast sagging option is uplifting of your breast by surgery called as mastopexy .
breast sagging occurs due to loss of skin elasticity , genetic cause.prenancy.
so in this surgery they will remove excess skin and reshape the breast .
but for which you should not have any chronic illness.
and if you want get pregnant u cant undergo . it may affect because breast will enlarge again .
and after surgery also you have to do exercise to keep breast uplifted.
so consult a plastic surgeon and discuss with them .