Doctors are conducting tests on the 94-year-old, CNN reported.
They told BBC News the tests so far have indicated there is no reason for alarm.
Officials have used similarly soothing language to explain previous hospital stays, but in those cases Mandela later turned out to have more serious conditions, The Associated Press reported.
Mandela has not appeared in public since 2010.
He spent 18 days in the hospital in December for gallstones and a lung infection, and has been admitted to the hospital three times in the past two years, according to BBC News.
Mandela served as South Africa's first black president from 1994 to 1999 after being imprisoned for 27 years.