Buckingham Palace said the monarch was seen by experts in London on Wednesday.
Buckingham Palace says the Queen has spent a night in hospital after canceling her trip to Northern Ireland.
A spokesman said the monarch went to hospital on Wednesday for a “preliminary investigation” but returned to Windsor Castle on Thursday.
The 95-year-old queen was seen by specialists at the private King Edward VII Hospital in central London. It is understood that his admission was not related to the coronavirus.
The spokesman said: “Following medical advice for a few days of rest, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returned to Windsor Castle at lunch today, and remains in good spirits. “
The PA news agency reported that it is understood the overnight stay was for practical reasons and that the Queen was back at her desk on Thursday afternoon and doing light duty.
The head of state’s stay was kept secret, but the Palace released a statement on Thursday evening after The Sun newspaper broke the news.
Vikas was not announced at that time as she was only going to appear for a short time. Protecting the Queen’s medical privacy was also a consideration.
She is said to have been disappointed at not being able to travel to Northern Ireland on Wednesday, and reluctantly heeded her doctors’ advice.
The country’s longest-reigning monarch, whose husband Prince Philip died just six months ago, has had a busy schedule since returning from Balmoral in early October.
She hosted a major global investment summit at Windsor Castle on Tuesday evening, where she looked bright and cheerful as she carried out her royal duties.