Short and bra cup size has nothing to do with back pain. Your cup does not support your boobs the support comes from the band. I too was having back pain, thought it was because I wasn't lifting upper body weights. Turns out my bra band was too big, and the cups too small. I went from a 30D to a 28FF and I'm 5'1". It took some used to a tighter band but the back pain went away.