Here's the thing most women will lose weight but their bra sizes may not go down at all, especially if they breastfed. What most women don't realize is when you gain weight in general your band size changes while your cup usually doesn't. when you have a baby though all bets are off. First i suggest you go get fitted to make sure you are wearing the correct size now. &0% of all women wear the wrong size bra and everyone should get refitted every year or so.
As for the back pain if it gets severe you may want to look into breast reduction surgery. I wore a 36D when i was 14 and weighed 100. now i'm a 38DDD since i stopped breastfeeding (my nursing bras were 42H)and i'm getting breast reduction surgery later this year.