The former Boyzone star is currently touring the UK but he has been feeling unwell and consulted a medic about his symptoms.
In a post on his Twitter page on Sunday morning, he wrote: "Feel like I have been run over. Time too (sic) see the Doc."
Keating took to the stage at the Manchester Apollo as planned that evening, but he returned to the site after the show to tell fans of his struggle.
He added: "To everyone that came to the Manchester show tonight. Thank you. U were an amazing audience. U carried me through till the end. Sorry that my flu effected (sic) the show. But we got through it together. Thank you."
Keating's tour has already been blighted by health issues - he injured his ankle just hours before a show in Liverpool, England on Friday night.