Britney's Son Hospitalized, 'Doing Fine'
"He's doing fine," the source tells PEOPLE. "They kept him overnight."
Jayden was taken to the hospital after suffering "a terrible allergic reaction. He had hives, was itchy and irritable," according to the source, who says that Britney Spears and her mother, Lynne, spent the night at the hospital with Jayden.
As for what caused the reaction, "They're just not sure," said the source.
Monday afternoon, Spears's team posted this statement on her Web site: "Jayden James Federline was admitted to the Southwest Mississippi Regional Medical Center on Sunday, November 9th. Doctors concluded he had a reaction to something he ingested. He is expected to be discharged tomorrow."
Just prior to the incident, Britney, Jamie Lynn, Bryan, Lynne and Jamie Spears were enjoying a fun afternoon at the family's Serenity mansion, the boys' first trip out of California with their mother since she lost custody.
A rep for Britney's ex-husband Kevin Federline had no comment.
Reporting by RUTH LANEY, JENNIFER GARCIA and MELODY WELLS
Who's your Favorite Star Under 35? PEOPLE.com and the People's Choice Awards are counting your votes! Click here to vote now!
See ALL of Britney's PEOPLE covers!