Ribs sprung, cracked, broken, withers process broken, neck out? All sorts of things.
Can also be saddle fit, as mentioned.
And are you tenting saddle blankets, making sure saddle is far enough back, making sure your cinch is not too long and pressing on nerves/blood vessels, that you are not cinching too tight to begin with?
Could also have something stuck in body, under where cinch is lying. Found 4 inch thorn stuck straight up in center of cinch area on belly one time.
Abscesses deep within muscles too will cause this.
Horses make me a better person.