Keith
This sounds like some sort of a “febrile convulsion” Infants have an underdeveloped hypothalamus which is the body’s thermostat. In a nutshell, babies and infants cannot regulate their body temperature as well as adults, and if suffering from a fever, their bodies can become so hot that they begin to convulse. If the body temperature is not reduced, this can lead to death.
Children who are teething often have a fever, and perhaps the child was also harbouring another infection, which led to an even greater rise in their temperature.
Rick