According to Wikipedia, 75-85% of women are wearing the wrong size bra. Those numbers seem really high, until you start thinking about it- weight changes and pregnancy are two of the most common reasons for change in one’s bra size. Many women buy the same size all their life, regardless of what has changed in their life, and wear bras that have outlived their usefulness.
A good bra is about more than esthetics- it will provide necessary support, thereby decreasing the load on your spine, particularly the lower neck and upper to middle back and the joints between the ribs and vertebrae.
As a chiropractor I have seen women with divots in their shoulders from wearing bras that were too small with narrow straps that dug into their flesh. Women who developed earlier than their peers may slouch to deflect attention when they are young and this can cause a snowball effect of compensations including vertebral subluxations and chronic muscle tightness.
So, go get fitted. Both independent lingerie stores and the larger department stores should have staff trained to do this properly- call ahead to be sure they do. A good bra will not be cheap, but it is worth it. And go get your back checked by a chiropractor.