Secretary of State Hillary Clinton was admitted to a hospital Sunday after doctors discovered a blood clot stemming from the concussion she sustained earlier this month.

Clinton, 65, suffered a concussion after falling early this month while she recovered from a stomach virus, her spokesman said. The clot was discovered during a follow-up exam Sunday, Philippe Reines said.

Clinton canceled a Dec. 20 appearance before Congress about a report into the Sept. 11 attack on the U.S. diplomatic mission in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans. Some critics suggested she faked her illness to avoid testifying.

She will spend at least two days at New York-Presbyterian Hospital. The former first lady and senator announced she would step down during President Obama's second term. Sen. John Kerry, D-Mass. will replace her.