I also experience post-shit dizziness and fatigue. Have never gotten shortness of breath though.
I read it can be if you shit large you lose alot of fluids and this loss of fluids can cause your blood pressure to drop. Once this happens I’ll eat a big meal and drink fluids and it quickly goes away.
But I usually have my bowel movements in the morning and having a large breakfast with enough fluids eliminates this problem. It’s only when I skip or have a small breakfast that I get these symptoms after bowel movements.