5 yr old with headache vomiting sore throat. no fever?

Call Peds: Needs checked for strep throat. Not absolute, but possible. Call your pediatrician. Many kids with strep will have nausea or vomiting.

GI Virus? : Most common reason is an intestinal viral infection. These are self limited lasting a day or two. Rarely strep throat can present this way, but there's usually a fever. If diarrhea develops then its a virus. If he doesn't urinate for >12° or develops a fever call your pediatrician.