RIHANNA RAN OFF STAGE OVER THE WEEKEND

RIHANNA RAN OFFSTAGE CLUTCHING HER STOMACH AT A CONCERT ON THURSDAY . . . BUT SHE IS OK:

Last Thursday night, RIHANNA ended a gig in Sydney, Australia by running offstage clutching her stomach. -It happened during her last song of the night, "Umbrella", which she performed as a duet with (her boyfriend) CHRIS BROWN. In fan-shot videos online, you can see her hold her stomach . . . hunch over . . . and run offstage.

--But Rihanna's people say she's totally FINE . . . she was just a little winded. A rep says, quote, "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. --"She is perfectly healthy and will be at her show [Saturday night in Sydney]." (--As of last night, there were no reports that Rihanna bailed on that gig. So, we're going to go ahead and assume it went on as planned.)

--For what it's worth, TMZ is reporting that there was an air-conditioning problem at the arena . . . and that is why it was so hot. (--Although . . . as you can see in the video . . . Chris Brown was wearing a SWEATSHIRT onstage.) -Rihanna's reps haven't elaborated any further . . . but one of the tour promoters told "The Sydney Morning Herald" that she DID collapse. He said, quote, "Rihanna experienced breathing difficulty and collapsed offstage. --"She received medical attention and has recovered. The problem seems to have been caused by a combination of overheating, dehydration and pyro[technic] smoke."