I was only a 34C prior to pregnancy so nothing too overwhelming and up until I was about 19 weeks gestation I managed to remain in my usual bras although they did start becoming uncomfortable from 16weeks.
I'm now almost 22 weeks and a fortnight or so ago I bought some new bras - I didn't get measured, just guessed - and just went up one cup size to 34D which fit really well.
However, a week after buying them they also became so, so painful to wear. The cup size is fine still but the strap that goes around my back is agonisingly tight and I'm left with red marks across me and irritated skin. It is so unbearable that I'm walking around with my bra on but not actually done up 
I'm going to get some more bras tomorrow and probably go up a step to a 36D but I feel annoyed because bras are so expensive - I can't be doing with replacing them every few weeks.
I was kind of hoping that boob growth would slow down and eventually stop.....am I kidding myself?? 