Hangonfi, do you ever have your bra strap ride up your back? Do you get red marks on your shoulders from the straps? When you put your bra on does it sit perfectly parallell to the floor and stay there?
99% of the time it isnt the cup size that is wrong its the back size. Many many people wear a bra with a back size far far too big for them. I'm not dainty, I wear a UK 14 clothing and weigh over 12 stone (168lb) but am only a 32 back, most shops want to put me in a 38 DD but that ends up in my bust and back hurting. I found Bravissimo stores, they fit bras differently to traditional shops and my 32GG bra fits properly, gives all the support fram othe back band, I no longer get back ache, I no longer get red marks on my shoulders and I can even do sitting trot in a pretty plunge bra and not hurt (used to be unable even in a sports bra). When I first put the 32 on it felt uncomfortably tight but an hour later It was so much better!
I'd put money on the fact that you are wearing the wrong back size, just remember that if your back size goes down your cupsize may have to go up to compensate.
Bravissimo have a very good fitting guide here: Perfect Fit Guide | Bravissimo