Kirchner, 60, "sustained the head injury August 12 and had tests at the time that showed nothing," spokesman Alfredo Scoccimarro said yesterday.
But new tests identified the chronic subdural hematoma, a type of hemorrhage, and "doctors ordered her to rest for a month," he said in a statement.
Scoccimarro said that Argentina's first democratically-elected female leader was not ordered to be on total rest.
He did not immediately say how much she would be working, but added that Kirchner would be receiving follow-up treatment.
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Kirchner has had several health concerns while in office described as cerebrovascular in nature.
In January 2012, less than a month into her second term, she underwent surgery to remove her thyroid gland only to be told that she had been mistakenly diagnosed with cancer.
During her first term in office, Kirchner on several occasions cancelled official appearances and events citing exhaustion.
Her husband, Nestor Kirchner, was a popular two-term president who died of a heart attack in 2010, but saw his wife succeed him as president.
Internationally, she has kept up her country's push for talks at the United Nations on the sovereignty of the British-ruled Falklands, which Buenos Aires claims.
Argentina, which was then ruled by a military junta under General Leopoldo Galtieri, invaded and occupied the islands in April 1982.
This prompted Britain to send a task force of 100 ships to recapture them in a war in which 649 Argentine and 244 British troops died.