They just feel like the muscles in your stomach are really tight. Sometimes with mine, they are tight enough that I have to slow down my breathing for a minute (since all the stomach muscles are tight, nothing can move down to allow for breathing quite as much). They last for a few seconds to a minute, and then go away. Sometimes I get a few about 5 minutes apart, and then they will stop for a few hours, or even a couple of days. They never hurt or feel sore or anything like that.