Hi kristi, seizure activity usually presents with stiffening and then jerking (a tonic clonic seizure) although it can also be shaking and jerking of one body part such as a foot or an arm (focal fitting) or epileptics sometimes suffer absence seizures where they look normal but won't respond ie. the lights are on but nobody's home! If your DS was limp in your arms, it is unlikely that he was seizing. It sounds like your DS was unconscious for a prolonged period of time and so may not be related to the breath holding at all.

How did you go at the doctor?