The Secretary of State has been undergoing treatment for a blood clot just below her ear that was reportedly caused by the concussion she suffered in mid-December. She is expected to make a complete recovery. NBC News chief science correspondent Robert Bazell reports.

Secretary of State Hillary Clinton was admitted to a New York City hospital on Sunday after doctors discovered that a blood clot had formed, the State Department said in a statement.

Philippe Reines, a deputy assistant secretary, said in the statement that the clot stems from a concussion Clinton sustained several weeks ago.

Reines said that Clinton, 65, is being treated with anticoagulants at NewYork-Presbyterian Hospital in Manhattan. She will be monitored there for the next 48 hours, he said.

“Her doctors will continue to assess her condition, including other issues associated with her concussion," he said. "They will determine if any further action is required.”

Dr. Roshini Raj, a physician at New York University Medical Center and a contributor to the TODAY show, says it’s not at all clear where Clinton’s blood clot is – which is important for understanding how serious her medical condition is.

“It’s a little murky,” Raj told TODAY. It is uncommon for a concussion alone to cause a blood clot. More likely is a blood clot elsewhere from lying in bed to recover from a concussion, Raj said.

Dr. Roshini Raj, an attending physician at NYU Medical Center, speaks with TODAY's Willie Geist about Hillary Clinton's blood clot complication, how common it is and what it may mean for her health going forward.

Related: Hillary Clinton recovering after fainting

Clinton suffered the concussion from fainting earlier in December. She had been sick for several days with the flu and had canceled a trip to Morocco where she was to officially recognize the Syrian rebels.

Brain injury doctors told NBC News said that although details haven't been made public, initial reports indicate that Clinton may have developed a blood clot in her lower limbs as a result of prolonged rest and inactivity after her recent concussion.

A deep vein thrombosis, known as a DVT, or a dural venous sinus thrombosis, could be two types of blood clots treated with anticoagulants, said Dr. Alex Valadka, a spokesman for the American Association of Neurological Surgeons. A blood clot could be dangerous if it breaks free and lodges in a vital organ, such as the heart.

Kevin Lamarque / Reuters file Clinton, photographed here on Dec. 6 at Dublin City University, canceled a trip to Morocco earlier this month after a bout of flu. She was hospitalized Sunday after doctors discovered a blood clot stemming from a concussion she sustained earlier in the month.

A deep vein thrombosis could be serious, but not necessarily life-threatening, and would require months of treatment with blood-thinning drugs, said Dr. Inam Kureshi, chief of neurosurgery at Hartford Hospital in Hartford, Conn.

"Usually hospitalization is more of a precaution," Kureshi said.

Slideshow: A political life Kevin Lamarque / Reuters Hillary Clinton's life has taken her from first lady to senator to secretary of state. Launch slideshow

It is possible that Clinton developed a blood clot elsewhere, including her brain. Doctors interviewed would not speculate about treatment or prognosis for the secretary of state.

Days after she fainted, State Department officials said she was at home recovering. Officials also issued a statement from Dr. Lisa Bardack of Mount Kisco Medical Group and Dr. Gigi El-Bayoumi of George Washington University that provided more information about the secretary's condition:

"Secretary Clinton developed a stomach virus, leading to extreme dehydration, and subsequently fainted. Over the course of this week we evaluated her and ultimately determined she had also sustained a concussion. We recommended that the Secretary continue to rest and avoid any strenuous activity, and strongly advised her to cancel all work events for the coming week. We will continue to monitor her progress as she makes a full recovery."

It wasn’t the first time Clinton passed out while sick with a stomach bug. As a U.S. senator representing New York, Clinton fainted in 2005 during a speech in Buffalo after complaining of a stomach virus.

NBC's Robert Bazell tells MSNBC's Chris Jansing that it is unclear how severe the blood clot could be for the health of Sec. of State Hillary Clinton, given lack of specific information about her condition on Sunday evening.

NBC's JoNel Aleccia and Isolde Raftery contributed reporting.