The two-year-old was admitted to the Southwest Mississippi Regional Medical Centre on Sunday after becoming 'vague and unresponsive'.
And he was kept in overnight for observation.
Doctors concluded he had a reaction to something he ingested, a rep says in a statement.
It is the first time Britney, 26, had taken the tot and his brother Sean Preston, 3, out of LA since ex-husband Kevin Federline, 30, gained full custody in July.
They were visiting family in Louisiana when he fell ill.