For yrs I always measured 34D (Debenhams, little indie shops, everywhere). If I wore 34D I got backache & indigestion. So I wore 36C instead which were great, but fitters got snooty about me preferring 36C so I don't like going to real shops.
My old bras became loose so I sewed them up to be tighter, but the cups became too small (sigh). I remeasured under & over,
I'd try a 32D and work from there. Even if you haven't lost weight, you've probably lost body fat. Ordering online from somewhere with free returns is the best bet because you can try things on in your own time and it's pretty obvious when they're wrong.
Whether the size you measure is a comfortable and flattering fit can depend on the brand. I'm 32GG, but wear 34F in a Triumph style that I like because the bands are less stretchy than the other brand I wear and I also prefer the shape they give me with the 34" band.