The announcement by presidential spokesman Alfredo Scoccimarro came after Fernandez underwent cardiovascular and neurological tests at the Favaloro Foundation, Diario Hoy reported.
Scoccimarro said Fernandez suffered the injury Aug. 12. He didn't say how she got hurt.
Voice of America reported initial tests had found nothing wrong but the new examination revealed a chronic subdural hematoma.
It wasn't clear whether Fernandez would continue to carry out her official duties or take a formal medical leave and relinquish power to Vice President Amado Boudou, VOA said.
The New York Times reported Kirchner, 60, had more tests after experiencing symptoms of a migraine during a routine exam for an abnormal heart rhythm.
Kirchner, in her second term in office, was incorrectly given a diagnosis of thyroid cancer last year.