Do you mean the kind of mild epilepsy resulting in "absences", where the sufferer momentarily appears to zone out? If it's this, I've never heard it called a thinking spell before.
Milva - "thinking spell" means precisely a short period of time (spell) where you appear to be thinking - i.e. petit mal. It is not a common expression to describe this condition or experience.
What I don't understand about that is: when someone has a 'petit mal' he/she is actually unconscious for a moment, so why "thinking spell"? Doesn't it sound illogical?