The Queen spent the night in hospital 'for some preliminary investigations'. This comes after she was forced to cancel her two-day trip to Northern Ireland following doctors' orders.
The 95 year old monarch, who was 'disappointed' after she was unable to attend engagements in Northern Ireland, was discharged on Thursday 21 October after she was admitted to hospital on Wednesday afternoon.
According to sources, she returned to some light duties at her home in Windsor Castle on the very same day.
"Quite understandably, a cautious approach is being taken by the medical team," they explained to the Daily Mail.
"Her Majesty was back at her desk this afternoon and undertaking light duties."
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"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 Buckingham Palace spokeswoman said.
The Queen's stay at the private King Edward VII's Hospital was her first overnight in eight years.
She spent a night at the private clinic in 2013 when she was treated for gastroenteritis - a viral tummy bug that can cause vomiting and diarrhoea.
The 'cautious' approach to the Queen's return to work, follows an intense programme of official duties, including 19 engagements since the beginning of October. She continues to carry out hundreds of work commitments each year - 30 years after most people have retired.
The Royal editor of the Daily Mail, Rebecca English tweeted, "The Queen sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future."
She further explained, "It’s not Covid related. At the age of 95, it’s sensible rather than serious."
The exact reason has not been disclosed as the Queen also has rights to medical privacy.
She added in the comments section: "She’s had a packed schedule of engagements in recent weeks including attending the opening of the Welsh Parliament and last night’s Windsor reception - and she has COP26 around the corner."
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The Queen, who has recently started using a walking stick at public events, has plenty more on her calendar including her Platinum Jubilee - which will celebrate 70 years on the throne - next year.
The UK will be given an extra bank holiday in June to mark the impressive achievement.
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