Sudden stomach pain after waking up with one episode of vomiting in an 8-year-old is most often due to a mild stomach infection (viral gastritis), indigestion from something eaten the previous day, or gas buildup, and it usually settles on its own within a day; less commonly, it could be due to constipation, food poisoning, or early signs of conditions like appendicitis (especially if pain becomes severe, shifts to the lower right side, or is associated with fever and repeated vomiting). For now, keep your child well hydrated with small sips of water or oral rehydration solution, avoid heavy or oily foods, and allow rest. You can give light, easy-to-digest foods once the child feels better. Seek medical attention urgently if vomiting continues, the pain is severe or worsening, there is fever, dehydration (dry mouth, less urine), or if your child looks very unwell.