This sounds like a silly question, but my back is killing me… I practice my recorders in our exercise room, so I generally sit on the exercise bench to play my bass (with floor peg). I try to be aware of good posture, but I think a chair with some sort of adjustable back support would reduce the pain in my aging back. So I wonder whether anyone here has found a good (preferably portable) adjustable seat for recorder playing. After all, there are cello stools and guitar chairs; why not recorder chairs?
Have you looked at one of the larger music stores like Thomann? They offer all sorts of possibilities of chairs and stools, many adjustable, some more portable than others. I prefer my collapsible IKEA Franklin Bar Stool for practicing, but stand when in rehearsal or performing (subbass).
Perhaps yours is a problem with posture. Have you tried a neckstrap, or raising or lowering the peg? Or getting someone else, preferably a recorder teacher, but perhaps anyone, to look at your body position while playing? The mouth piece, instrument height etc. should adjust to you, not the other way around.
Also perhaps there is some tension or too much body stiffness while playing? A second set of eyes might be helpful.