Hi. Thanks for your query. Read the history of your Son and understood it. A child of nine and half usually do not get muscle cramps, so there has to a definite reason for the pain after workout. You have given a history of hernia, but not mentioned whether it is operated or not. This can be a reason for the pain in abdomen and vomiting. If the hernia is not operated, your son might be getting obstruction in the hernia that is causing the pain. Another reason can be a testicular torsion, as this can easily happen when a child has a congenital hernia. There can be another reason of having either the intra-abdominal lymph nodes or something like appendicitis that has got reactivated and causing the symptoms. I would suggest you the following as your son is just nine and half years old:: First of all get an opinion of the Pediatric Surgeon ASAP. Get blood and urine. Ultrasonography and color doppler of the scrotum and abdomen.. A clinical examination and its correlation with the complaints can give a diagnosis. Till then keep him nil orally. I am sure the diagnosis will be made and proper treatment instituted urgently.Remember that the children can go into dehydration pretty fast.