I would have your son checked for infantile spasms. These are basically infant seizures. Most of the time infants grow out of them or some times they can turn into a seizure disorder as the child becomes older.
It is not uncommon for the child to shake or "stare into space" and then go back to being perfectly happy.
I would keep an eye on it. Take him to the pediatrician and maybe push for an EEG just to be sure.
I have experience with this because my niece had the same thing when she was younger.
It's not the end of the world so I don't want you to be afraid, but I would want to know so in the future, you and your pediatrician could keep an eye on it.
Watch also now for fever and whether or not that brings the shaking on. Bristal's infantile spasms were exacerbated by fever.
If your son does have infantile spasms there are certain things you can or cannot do when he is going through them so be sure to discuss them fully with your pediatrican.
Good luck and God Bless. Let me know how things go.