The dose may have been inadequate or barely adequate. They fear overdosing. It may or may not be a sign of a serious infection. Fevers make parents crazy. They fear the possibility of a febrile seizure most, it seems.
High fever equals seizure.
Febrile seizures can occur with only a minimally elevated temperature. Febrile seizures usually occur when the fever goes up rapidly. That's why we usually get that ambulance call between 2am and 5am or late in the afternoon after a nap. They usually outgrow this propensity towards febrile seizures around the age of 3 years. Do not wrap him in heavy blankets.