I'll jump in because I had this happen when I went from M&E (fatter) toherbivore12 wrote: It's more slender in outside diameter than most other wooden flutes I've
played. ...combination of the McGee's slender body and hole layout
Olwell (narrower). I got first cramps in the top thumb and then in the top
fingers across the back of the hand. I thought it was the diameter.
So I got some of that clear plastic tubing from a hardware store and made
a home-made bo peep out of it. I used it between the top hand pointer
finger and the flute. On and off for a few months.
But then the main thing that cured the problem was doing long tones on
the new flute (without the bo pep if you can) because it seemed that it
was the hands trying to compensate for something out with the
embouchure.
As a test I'd tried to finger some tunes without making a sound. Not a pain
or a cramp anywhere, no bo peep. That clinched it for me, it was the
embouchure.
Hope that helps, good luck.
Cheers, Lesl