Last night, i think i was half sleep/awake when i flinched or flailed my whole body once. did i have a seizure? what could that've been? was it dream?

Unlikely seizure: Your description is much more likely to have been either an hypnopompic or hypnogogic jerk. These are normal phenomenon and you've likely done them lots of times but this time you were aware of it. Hypnopompic jerks occur as we are in a twilight state and awakening. Hypnagogic jerks occur in twilight state upon falling asleep.