I recently went back to Rigby & Peller, the specialty shop where I had my first professional bra fitting, and I'm officially one cup size bigger than at my last fitting three years ago. It isn't the biggest difference, actually, because my chest circumference remains the same, and a properly fitted bra often has the effect of making one look a little smaller under clothes regardless. Nevertheless, I am a bit mystified by how my size can change when most of my measurements didn't, but bra sizes are mysterious. In hindsight, I should have gone in sooner. I've been shopping Nordstrom sales for new bras, and I'd been marveling at how difficult it was to find a good one even when I knew my proper size. I was chalking it up to different brands and designs having considerable variation, which is true, but the main factor likely was that I no longer had my correct size at all. Oops!
I am a bit distressed because I bought a few relatively pricey things this year that won't work as well with this change: a bikini in March and a sports bra in June. I've been using the latter frequently, and the size mismatch isn't too serious there, at least. It has some visible hallmarks of a not-quite-fitting bra when worn, but it's comfortable and keeps things stable, if you will, during my workouts. It serves its purpose and probably doesn't demand a replacement anytime soon. I have a few rarely used items, including a beautiful Chantelle bra that I bought last December (in the shade pictured above), which I might consider putting on eBay now.
I highly recommend getting professionally fitted. It can be a magical experience, as Nicole at Writes Like a Girl recently reported. (Cassie at The Minuteglass is also no stranger to a the joy of having the right fit.) For those who are able to get spend-y and live near a Rigby & Peller, they do an excellent job. They take a lot of guesswork out of things and will bring you a selection of things to try, based on your preferences, once they've figured out your size. Essentially everything their sales associates bring to me is a perfect or near-perfect fit, and all that after figuring out my size by just taking a glance at how I looked in the bra I wore to the store. (I was wearing something very close to the right size each time, though.) They are very pricey though, especially if you're in a less common size. They stock a few bras in the $70-$80 range, but almost everything that worked for me this time was around $120. The cost per wear analysis should be favorable though: I tend to stick to a very small collection of two or three bras in frequent rotation, and I get a solid two and a half to three years of wear that way. I handwash my bras in The Laundress delicates wash, but I think Cassie mentioned getting similar longevity by machine-washing.