Is it normal when you start stretchsitting to experience a little trouble breathing? Kind of like there is some pressure in your lungs?
Stretching your torso, and in particular your abdominal wall, has a similar effect to toning the abdominal wall. There is a little pushback to the lungs expanding in the direction of ballooning out the belly. If this gives you some challenge in breathing, it's time to work on your ability to expand your lungs in the direction of filling out yor chest cavity. A similar challenge happens when people first learn to do the "inner corset" and the abdomen is similarly less available for expansion of the lungs. You don't want to only be able to breathe in the abdomen and run out of breath every time the abdomen is stretched or engaged. The good news is if you persist in breathing deeply into your chest, the muscles between your ribs wil stretch and become more available to deeper breathing.