It was later confirmed by Buckingham Palace that the 95-year-old monarch was admitted to hospital on Wednesday afternoon for "preliminary investigations" and spent the night there.
"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," a spokesperson said.
The news has left many royal watchers rattled, as Prince Philip died in April aged 99 and there are fears the Queen's health may be in decline.
It's understood that the royal family are devising new strategies to support the ageing monarch as she continues to carry out public duties.
The Telegraph UK reported that Her Majesty will now be accompanied by one of her children or grandchildren for all public engagements.
This is to prevent the public from being "let down at the last minute" if the Queen has to pull out of an engagement for health reasons.
The outlet also reported that Prince William and Prince Charles have increased their workload behind the scenes, to take some of the pressure off the Queen.
Sources close to the royal family have also told The Times that the Queen will be cutting back on her public duties, as she is "knackered" after almost 70 years on the throne.
The Sun also reports that palace staff have been ordered to cut back the monarch's busy schedule, which has been packed with an "exhausting" number of events until now.
It's understood that the Queen will still attend the UN climate change summit in Glasgow next month, but that her schedule will now include plenty of breaks and time to rest.
This article originally appeared on our sister site, Now To Love.