Having sudden abdominal pain after straining during a bowel movement; what could it be?

Many causes. Excessive straining may just cause pain from overstretching/overpushing of the bowels. It may also be that you stressed your abdominal muscles. Sometimes, if you are known to have a hernia (umbilical or groin), you may push it out and it may get trapped causing severe pain. This is an emergency and needs to be dealt with in er. Some patients especially elderly may end up with spine fractures.