"Following medical advice to rest for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits," the spokesperson said.
It's understood the Queen stayed in hospital overnight for practical reasons, rather than any major medical concerns.
Reports are that she was back at Windsor, at her desk and working by the following afternoon.
Her Majesty was supposed to travel to Northern Ireland on Wednesday afternoon but her trip was cancelled on doctor's orders.
"The Queen has reluctantly accepted medical advice to rest for the next few days," a spokesperson said on Wednesday.
"Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland, where she had been due to undertake a series of engagements today and tomorrow.
"The Queen sends her warmest good wishes to the people of Northern Ireland and looks forward to visiting in the future."
The cancellation came after weeks of events and engagements for the 95-year-old monarch, who has kept a busy schedule even after husband Prince Philip's death.
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She took time off over the summer for her annual trip to Balmoral, in Scotland, but returned to work soon after travelling back to Windsor.
It's expected that the Queen will remain at Windsor Castle for a few days to rest and recover after this latest medical scare.
Though royal commentators like ITV's royal editor Chris Ship reported that the news "is 'no cause for alarm' about the Queen's health", some royal watchers are fearful.
At 95-years-old, Her Majesty is Britain's oldest living monarch and has faced several health scares in the last decade.
However, all reports are that the Queen is in good health and there are no ongoing concerns for her wellbeing.
She is expected to return to public royal duties in the coming days, with the support of other senior royals like Prince Charles and Princess Anne.
Both have stepped up to support their mother since Prince Philip's death and have been photographed with her at several royal engagements recently.
This story originally appeared on our sister site, Now To Love.