I experience very specific right sided cramping/tightening in abdomen if straining during  a bowel movement.  The spasm subsides quickly once I realize that I was straining & stop so it is not excessive strain, just more pushing.  This does not happen any other time. It does not happen with every bowel movement.  I am a 44 year old female & relatively fit.  The same symptom does not occur with any abdominal workout.
Hi, It seems that you might be having constipation and while passing hard stool there might be spasm producing this type of tightening. Take more fiber in your diet. Take plenty of water and liquids. Take mild laxative at night for few days. Ok and take care.