The singer ended her Australian concert with her hand clutched over her skintight black corset and tights and fell backstage just seconds after starting 'Umbrella', which was her final song of the evening, leaving her rumoured boyfriend Chris Brown to step in and conclude the performance.
Doctors suspect that the singer, who was carried away on a stretcher to hospital, suffered from dehydration caused by her outfit over-heating on stage.
London based Dr Margaret O' Shaunessy told The Daily Star Sunday: "The outfit could have been too tight and impaired what's known as the venous return ? the blood flow back to her heart.
"That would affect her cardiac output and make her faint. Also the outfit would see her sweat more, leaving her body dehydrated."
A spokesperson for the singer said: "It was very hot at the venue, so after Rihanna's set was over, she went off stage and needed to sit down, catch her breath and have some water."
The singer's tour of Australia and New Zealand is due to end on 11 November.