Only talking from personal experience, but sometimes after a BM I am suddenly stronger and feeling altogether better, but sometimes I am suddenly weak, shaky and out of breath. The suggests strongly to me that the state of (part of?) the bowel has a strong influence on our (or at least my) symptoms, especially PEM. When you have a bowel movement, the environment in different parts of the bowel changes. It may be to do with pH, or gut flora, or other things. I only know what I experience.