What's wrong with my gerbil?

he could have had a seizure. mine once had one, but thats because he was super old for a gerbil (over 4 years!) and was about to die. dont worry about him unless it keeps happening. if not, hes should be fine. just keep a close eye on him. if it keeps happening, call a vet and ask what they think. they may want you to bring him in so they can check him.