The singer was the runner-up in Saturday's Britain's Got Talent final, losing out to dance group Diversity.
Boyle was taken to the Priory by paramedics after a source claimed that she had been "acting strangely" at her hotel, The Sun reports.
A Britain's Got Talent spokeswoman said: "Following Saturday's show, Susan is exhausted and emotionally drained.
"She has been seen by her private GP, who supports her decision to take a few days out for rest and recovery. We offer her our ongoing support and wish her a speedy recovery."
A show source said: "It's very tough, Susan is emotionally drained - she gave it her all and is absolutely shattered.
"Simon Cowell spoke to her backstage on Saturday night and told her she had everything going for her - a record deal, an American tour. But it's up to her, she has to see what she wants.
"We do realise that we have a care of duty to look after her."
Last week, Boyle's brother reportedly claimed that producers of the talent show should have looked after the singer more.