Mariah Carey -- I'm Going Dark Tonight ... Concert Cancelled

Mariah Carey's tour is already rocky and it's only been a week ... she's just cancelled Wednesday night's Vegas show.

Our Mariah sources say the singer has bronchitis and her doctors have advised her not to use her voice.

We're told so far only Wednesday night's show is cancelled and she has a couple of days off until the next scheduled performance. But it's uncertain she'll be able to perform.

The cancellation came at the last minute. People have flown in for the show and we're told many have arrived at the venue, where they saw the sign that says "Tonight's performance has been cancelled and will not be rescheduled."