banananutmuffin ok, this definitely sounds like a case of breast tissue migration then! It sounds like most likely you need a 28 band, 34 bands have to be huge on you!!! What is your bust measurement? I discuss how to take it
here (there are a few different options if especially you're in a ill-fitting bra).
Using sister sizes a 34D has the same volume as a 32DD so in a 28 band you'd need a 34D=32DD=30E=28F (UK sizes). You might find, though, that you actually need a larger cup size than that because a too big band can mask a too small cup size. Additionally, when you put on your bra, make sure to bend forward and scoop all of your breast tissue into the cups. This will help with breast tissue migration.
free1 - Yep, breast tissue migration is very much a real thing! Women often think that they have extra back fat/underarm fat when in reality it's a combination of an illfitting bra (too big back, too small cups) and not putting on a bra correctly (you need to lean forward and scoop all of your tissue into the cups).
If you go from wearing ill-fitting bras to well-fitting bras you might actually find your cup size go up over time (and sometimes your back size might even go down).
If you need any help with size/fit let me know!