I vaguely recall reading this book some fifteen years ago, but would give good odds it was old when I first read it (but not yet yellowing). Certainly the setting felt old. It might also have been British in setting and/or publisher, especially if it was old; a few of the kids books we had were ones originally from my (British) grandmother, either from when she was young or when my mother was.
The main character was a young girl in a moneyed family. She discovers (on her own, I think, not told by someone else) that she has magic, mainly (entirely?) expressed through the ability to fly. I distinctly recall a description of the sensation... feeling like liquid gold in her stomach, or something like that, and gradual weightlessness, possibly giddiness? I think it got repeated a few times, it definitely stuck. She got caught doing this while falling/flying down the stairs, I think, by parents or a nanny. Not the first flight, some time after that.
It turns out she's got fairy ancestors or parents or something – I can't remember whether it was a traditional "changeling" situation or a "blessing" or something else – and she has to choose between keeping her magic or keeping the family/home/world she knows.
I've also got a bit of extremely vague imagery from it that I can't shake, something about a decorated box or a dollhouse?
Fairly trite stuff, yes, but it keeps popping back into my head for no apparent reason, and it has me curious. Parents sold most of those old books years ago, so I can't just comb the shelves.