A spokesperson added: ‘Following medical advice to rest for a few days, the Queen went to hospital on Wednesday afternoon for preliminary investigations, returning to Windsor Castle at lunchtime today (Thursday) and stay in a good mood. “
On Tuesday evening, Her Majesty attended an event at the castle, where she was seen alongside Princes Charles and William, and Prime Minister Boris Johnson.
The 95-year-old monarch also shook hands with Bill Gates and US climate envoy John Kerry.
On Wednesday, the palace said the Queen had “A medical opinion reluctantly accepted to rest for the next few days” and would not travel to Northern Ireland.
He added that His Majesty was “disappointed” that he could not make the trip and was “looking forward to visiting in the future”.
She was to attend a service marking the centenary of the partition of Ireland and the formation of Northern Ireland.
The monarch has had a busy schedule since returning from Balmoral in early October.
Sky royal correspondent Rhiannon Mills said: ‘In many ways it’s probably not surprising that they told her to rest as the past two weeks have been incredibly busy considering that she is 95 years old.
“Yes, she has all the best help around her and she leads a very privileged life, but for the past 95 years she has had a very busy work schedule. “
Recently an aide said the Queen believed “You are as old as you feel” and feels too young at heart to accept the main award for “Oldie of the Year”.