In addition to the pain in the stomach area, many men also experience severe nausea. This is thought to be caused by a huge rush of sympathetic nervous system discharge. So the body’s tolerance level for that rush determines whether that person will be someone susceptible to actually vomiting when kicked in the nuts.
So bottom line, getting kicked in the balls = severe pain in testicles + severe abdominal pain + quite a bit of nausea and possible vomiting.
In many places of the world, it is actually considered sexual assault to kick a man in the testicles intentionally in non-self defense situations, due to the above potential medical problems that can result in such an act.