Clinton spokesman Philippe Reines says her doctors discovered the clot during a follow-up exam Sunday. Reines says Clinton is being treated with anti-coagulants. She was admitted to New York-Presbyterian Hospital so doctors can monitor the medication over the next 48 hours.
Reines says doctors will continue to assess Clinton's condition, "including other issues associated with her concussion."
Clinton, 65, fell and suffered a concussion while at home alone in mid-December as she recovered from a stomach virus that left her severely dehydrated. The concussion was diagnosed Dec. 13 and Clinton was forced to cancel a trip to North Africa and the Middle East that had been planned for the next week.
Clinton was also forced to cancel Dec. 20 testimony before Congress about a scathing report on the Sept. 11 attack on the U.S. diplomatic mission in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans. The report found that serious failures of leadership and management in two State Department bureaus were to blame for insufficient security at the facility. Clinton took responsibility for the incident before the report was released, but she was not blamed.
Lawmakers at the hearings -- including Democratic Sen. John Kerry, the Senate Foreign Relations Committee chairman who has been nominated by President Barack Obama to succeed Clinton -- offered her their best wishes.