Bladder or fecal spasm? Sounds rough either wsy. I’ve never monitored my oxygen levels except when I had COVID. Maybe make sure you practice good, deep breathing techniques during spasms for the sake of your oxygen and also because it will give you a better sense of self-control. I get horrible back pain and back spasms when I’m getting close to having a bowel movement. I think the movement in my intestines presses on my vertebrae and/or back nerves or muscles. Fortunately women usually poop really fast compared to men so it doesn’t last long.