I grew up in an era when you were fitted for bras. I was taught that if the back hikes up, the barrel of the bra is too large. Your cup size may be staying the same, but the circumference below (ribcage only) has probably shrunk. I don't know in your size, but generally you buy a bra with a letter cup size larger as you go down in ribcage/barrel measurement (at least in the olden days). For instance, a 32D used to equal a 34C (but not always). Bras seem to be made a little differently now and each has its own quirks, but you should be able to solve the back hiking up with a smaller barrel measurement.
Sadie "hates to buy bras" Kate
PS - Let me tell you about the last business trip when I arrived and my luggage didn't. And I traveled in a casual skort.
