Her husband, the Prince of Wales, tested positive for the virus on Thursday but Camilla, 74, carried out three public engagements on that day after testing negative.
Prince Charles met with the Queen two days before he tested positive.
Last week Buckingham Palace said the Queen had no Covid symptoms and on Monday said it had nothing to add.
It is the first time the duchess has caught coronavirus, while Prince Charles has the virus for a second time.
A Clarence House spokesman said they would follow government guidelines.
