Valerie Trierweiler's chief of staff, Patrice Biancone, confirmed Sunday the 48-year-old journalist who lives with President Francois Hollande has been hospitalized since Friday.
Biancone said she was hospitalized for "rest and a few tests" and is expected to leave the hospital Monday.
Hollande has never married. Rumors have long circulated that he might have a lover.
The magazine Closer published images Friday showing a bodyguard and a helmeted man it says is Hollande visiting French actress Julie Gayet.
French media face strict privacy laws, but in recent years have chipped away at the tradition of ignoring the private lives of public figures.