U.S. Secretary of State Hillary Clinton has been admitted to New York-Presbyterian Hospital after doctors discovered a blood clot while performing a follow-up exam for a concussion she suffered earlier this month.

At a press conference early Monday about the September attack on the U.S. mission in Benghazi, Libya, Senate Homeland Security and Governmental Affairs Committee Chairman Joe Lieberman expressed his concern.

“We just want to say how much Secretary Clinton is in our prayers this morning and hope she recovers rapidly from this health problem,” he said. “She’s not only been a great secretary of state, she is a personal friend.”

Clinton has been ill in recent weeks with a stomach virus that forced her to cancel travel plans and public appearances, including a Congressional hearing on Benghazi. On December 21 she fainted, fell and suffered a concussion.

This is not the first time Clinton has suffered from a blood clot.

During a 2007 interview with the New York Daily News, Clinton said she got a blood clot when she was first lady. She told the newspaper she experienced a “terrible pain” behind her right knee as she campaigned on behalf of New York’s Democratic Senator Charles Schumer.

Clinton said the 1998 incident was “the most significant health scare I have ever had.”

Clinton plans to step-down in January, after U.S. President Barack Obama is inaugurated for a second term. She has visited 112 countries during her tenure at the State Department, making her the most traveled secretary of state in U.S. history.

Officials expect her to remain at the hospital through Tuesday.