Camilla is said to have been in "quite a lot of pain" in the last week, after Queen Elizabeth II's death.
It is believed she suffered the injury prior to the late monarch's death – but has taken it upon herself to "get on" with the job of supporting her husband.
The King has understandably had an emotionally and physically gruelling week, but his wife has remained an "absolute trouper" throughout.
The 75-year-old faces more days of hard work with the ailment, ahead of Queen Elizabeth II's state funeral on Monday.
King Charles III and Queen Consort Camilla leading the Royal Family in grief
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Queen Consort Camilla and Kate, Princess of Wales
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The Telegraph reports: “It is unfortunate timing to say the least but she’s been an absolute trouper."
The King's spokesman said: “We won’t comment on medical conditions.”
It comes as the royal couple are due to visit Wales on Friday – King Charles' first visit since he ascended to the throne.
They will attend a service of prayer and reflection at Llandaff Cathedral before receiving condolences at the Welsh parliament, the Senedd.
He will later hold a private audience with Welsh First Minister Mark Drakeford and the presiding officer at Cardiff Castle.
A protest against the monarchy is expected outside.
The King will then attend a reception hosted by the Welsh Government, before returning to Buckingham Palace in the evening to host faith leaders in the Bow Room.